I have dual citizenship in the USA and United Kingdom. I am planning
to travel around South America this summer, and I have learned that
certain countries (such as Brazil) charge significantly more for visas
on US passports than on EU passports. Thus, I would like to use my
British passport abroad. I know that I must leave and enter the US on
my US passport, but will it be a problem if, for example, I leave the
US bound for Argentina on my US passport and return from Mexico with
no other stamps in my passport (because I will have used my EU
passport for all international travel in between)? And when they say
you must leave the US on your US passport, does that mean I must use
it to enter my first destination country, or just that I must show it
to the airline agent upon check-in for my flight departing the US?

With dual citizenship, you are one individual with two entirely
separate sets of documentation.  For example, within the US, if you
wish to present UK/EU identification, you should also have valid US
documentation for residency and work permit.  That could be a problem
for you if you wished to reenter the States using your EU passport. 
IF you have that, of course you can travel from and return to the
States as a UK citizen. Maybe you can secure such documentation.
It is unclear from your question whether you "know" (perhaps for the
above reasons) or just have heard ("... when they say ...") that you
must enter and leave the States on your US passport. US Emigration
does stamp US passports, and will look for a exit stamp when you
reenter.  To what extend they check other stamps to follow your trip
...?  Probably post 9/11 more than before.
I (no expert, and this is no profession advice) would go ahead and pay
the higher fee for a visa to a US citizen unless you can secure the
documentation as a valid UK citizen resident in the States.
I thought through some possible other scenarios, but they could be
problematic at every border.

It is clear on the state department website that I must enter and
leave the US on my US passport, and I plan on doing so. I'm just
wondering about the travelling in between and whether or not it will
be problematic. As I am a natural born US citizen I would not be
interested in any extra documentation to complement my UK passport, I
only recently got it because my mother is a UK citizen. It seems that
it wouldn't be too much of a problem since it is perfectly legal to
have both passports and use them....I'm just looking for a secure
answer. But thanks for your comment!

Hi emarwood,

I'm also a dual passport-holder, one of them being a US passport. I
have travelled in and out of the US using both, like you are planning
to do. I have had no problems entering the US with no other stamps on
my passport.

As for when you leave the US on your US passport, you simply need to
show your US passport at the check-in and not at your first
destination country.

I hope this is helpful.

Best regards,
Rainbow
Google Answers Researcher

I have been reading the State Dept website on passports.  Of course,
it does say that one must present the US passport on leaving and
entering the States, but this  - it seems to me -  is merely an
explanation directed at US citizens.  You are both.  It is a little
schizophrenic, you are a Brit when you use the UK passport, hence my
explanation about having docs for residency etc.  At that moment, you
can't just say:  "Oh, but I'm also an American."  And vice versa in
the UK, if were to use the US passport.
Anyway, insofar as you do not need a visa to enter the US as a Brit 
(which I think is still true after 9/11, check) I was incorrect
before.  When you present the UK passport, at that moment you are not
an American and there is no requirement to us a US passport.

As someone who has both Canadian and UK passports , I take a very utilatarian 
approach to which passport I choose. If applying for a foreign visa ,
I will use the passport which is cheapest. When travelling to the US ,
I will use my
Canadian passport as this often entails less hassle at US customs (US
custom /immigration officials operate like robots memorising and
carrying out to the exact detail what appears to be hundreds of
regulations)
Thus when going to the US from the UK , I show my Canadian passport on
entry and my British Passport when I leave the US. (This is to
minimise any hassle when I get back to the UK) This has a tendancy to
confuse the US immigration officials as I do not have an entry
immigration form.
DPRK007

Myoarin is offering poor advice. You are always considered a citizen
of both nations no matter which passport you are presenting at that
moment. However, US State Department regulations require that any US
citizen returning from a foreign nation must present their US passport
to enter the United States, regardless of any other citizenships that
person may hold. You should never use your UK passport to enter the
United States, as you are a US citizen. When they say "leaving" they
only mean that you may be required to show your US passport at
check-in, because US citizens are required to carry their US passport
any time they are abroad (again, regardless of other citizenships).
Your best bet is to not confuse the immigration agents any more than
the bare minimum necessary. Any time you are dealing with US
immigration, present yourself as a US citizen (which you are). If
there is some confusion/complication (i.e. you are traveling to a
country that requires a visa for US citizens but not for UK citizens,
or in your case, perhaps the lack of stamps) then it may become
necessary to show your UK passport to US immigration and explain your
situation to them. Make it clear that you are 100% a US citizen, who
also happens to be considered a British citizen. For more information,
you should see this excellent FAQ: http://www.richw.org/dualcit/

Wordman-ga,
Thanks for showing that very good site. To a certain extent, I would
stick to my claim that the State Dept. is directing its statements
purely to US citizens, ignoring the possibility of dual citizenship. 
This is something that site mentions a couple of times.
I certainly agree with you that it is best to avoid confusing the US
or other nation's passport controls by showing a passport without a
visa for a flight to a country where one is required, of vice versa. 
Especially after 9/11!!
It is rather intriguing that although most countries would like to
avoid the dichotomies of dual citizenship, the US has as a result of
the Supreme Court decisions had to greatly liberalize its attitude.
THanks again.

Wordman
I found your web site regarding the joys of dual US/some other country
citizenship facinating. I am certainly no expert in this subject.
However it would make sense that a US citizen with another citizenship
should show his/her
US passport to US immigration officials when returning from a trip abroad.

But then I am thinking that every case may not be quite this simple.
Let us take a hypothetical example:

1. Jimmy is born in the United States and grows up in the United States
   to UK born parents. (He is therefore a naturalised American citizen)

2. When Jimmy is eighteen he applies for and gets UK citizenship. He also 
   procures a UK passport.

3. Jimmy is interested in visiting country X. Country X does not 
   have diplomatic relations with the United States. However country X
   does have diplomatic relations with UK and Canada.

4. Jimmy requires a visa to visit country X. He contacts a tour operator
   in Canada which specialises in tourist trips to country X. The Canadian 
   tour operator obtains a visa for Jimmy to visit country X using his UK 
   passport.

5. Jimmy asks the Canadian tour operator if he should take his US passport 
   with him ( as well as his UK passport) when he goes to country X. They   
   advise that perhaps he should leave his US passport behind as relations 
   between the US and Country X are a bit touchy.

6. Jimmy visits country X and has a great time. While entering and exiting he 
   has no problems with immigration officials from country X as all his travel 
   documents are in order.

7. Jimmy however realises he is not sure what to say to US immigration  
   officials when he returns to the US.

I see three possible outcomes:
 - Jimmy explains he is a UK citizen "visiting" the United States.
   The immigration officials accept his story and let him through.
 - After showing the immigration officials his UK passport , Jimmy explains he 
   is also a US citizen , but left his US passport behind on advice of the 
   tour operator.
 - Jimmy explains he is a UK citizen "visiting" the United States. However
   having searched his luggage , the immigration officials find documents 
   revealing that he is also a US citizen living in the Uniteds States.

  I am very curious in the last two scenarios if Jimmy has actually committed
  an offense. Also what would the repercussions be (for Jimmy):
  - A slap on the wrist?
  - A fine?
  - A Jail Sentence?
  - "Deported back to the UK"
  - "Deported back to country X"
  - Stripped of his US citizenship? 
  - not allowed out of the immigration area of the airport?
DPRK007