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THERE ARE DIFFERENT TYPES OF VISAS
Turist; businessmen or work visas; student; retired people visa; transit visa if you are going to another country and you cross mexico; amd cruise visas
with your u.s. passport you can cross to mexico by ground as a turist, you just pay a $20 USD fee
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Let me check the requirements for the other visas
For a turist visa, a U.S. citizen can stay up to 180 days, just showing his US Passport on the border
The same for a transit visa
If its a business visa, keep with you a letter with a petition asking you to travel to Mexico for work or business purposes; it can be made by a company in Mexico, a chamber of commerce, etc....
Mexico and U.S. has a lot of agreements, almost in all cases you just need to present your U.S. passport, the retired people visa american citizens need to present a letter and a balance from their bank accounts; so they can prove to mexican goverment they have a permanent income, in case you want to live in Mexico for a long period of time, in San Miguel de Allende, Los Cabos or any other place.
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