Does a us citizen need passport to fly within Mexico?

No, a US citizen does NOT need a passport to fly within Mexico. As long as you are entering the country for tourism purposes and staying less than 180 days, you can enter Mexico with just your government-issued ID and proof of citizenship (such as a birth certificate or naturalization certificate). If your stay is longer than 180 days, you must apply for a Mexican Tourist Card (FMT) from the Instituto Nacional de Migración (INM).