I left Cancun on Monday for a quick trip to Florida. I have an FM3, sort of a Mexican "Green Card", and that involves a quick trip to Immigration at the airport to get my documents stamped. There was a couple in line behind me who did not know they had to keep their FMT, which is the bottom of the Immigration Form that you fill out on the plane and present it to the Immigration Officer, the first step in entering the country. He or she stamps the form, tears off the bottom half and hands it to you. YOU MUST present this form to the airline ticket counter when you check in to leave. If you do not have it, you have to go to Immigration. The guy in this office will give you a form which you take to the Banamex (bank) just outside the Imm office, and you must PAY THE BANK $525 PESOS. The bank stamps the form indicating that they have received the money for the fine. It is due in pesos but the bank will accept dollars at eh current exchange rate and give you any change in pesos only Then you go back to the Imm office and the officer gives you a form to give to the airline. This is a big pain in the neck and a fairly new policy, that of paying a COSTLY fine. When the Imm guy told me to send in the next couple I told him they did not have their forms and he told me the drill. He did tell me that one of them could go stand in line at the bank and the other one could fill out the papers, then take them to the one in line. The line at the bank was pretty ugly so this was a time saver. BOTTOM LINE - DON'T LOSE OR FORGET THIS FORM. BEST ADVICE IS TO PUT IT INSIDE YOUR PASSPORT SO YOU DON'T MISPLACE IT.