U.S. Citizen Passport Requirements
after June 1, 2009

The Mexican government does not require a passport, just picture identification, like a driver's license to enter.

June 1, 2009, the U.S. government, through the U.S. Customs and Border Protection agency is requiring a passport for a U.S. citizen to reenter the USA. Actually, they can't keep a U.S. Citizen out, but it can be a hassle for you to prove you are a U.S. citizen.

Please see the link to the U.S. government website, for information on the various types of passports and travel documents accepted by Customs, below.

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Update, November 02, 2010
Nogales (AZ) International Newspaper


Unspoken rule allows passport-free border crossings by Wren Abbott

Their only penalty: a minor scolding from a U.S. Customs and Border Protection (CBP) official, who urged them to get passports.
Click to Go To:
Full Story at Nogales International or For a Copy

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If you need to go to Mexico immediately, you can go to the US Post Office and apply for a U.S. passport card ($55) and use your receipt and driver's license until you get your card.
Also see Story Above

http://www.getyouhome.gov/html/lang_eng/index.html