Bacterial isolates obtained from fecal cultures of U.S. adults in Guadalajara, Mexico, with diarrhea developing during the summers of 1987, 1988, and 1989 were tested for in vitro antimicrobial susceptibility. No resistance was seen among 220 enterotoxigenicEscherichia coliisolates nor among 89Shigellastrains to aztreonam, norfloxacin, ciprofloxacin, gentamicin, or furazolidone. High level (>1000 µg/ml) resistance to trimethoprim/sulfamethoxazole (TMP/SMX) was found in 7% ofE. coliand 3% ofShigellastrains. Susceptibility patterns of 27E. colistrains derived from fecal cultures of Mexican infants in Guadalajara with diarrhea during the same time period showed similar results, possibly reflecting the presence of a common microbial reservoir. The study serves as a baseline for newer antimicrobials (fluoroquinolones and aztreonam) where no resistance is currently seen and provides evidence of the continuing value of TMP/SMX for therapy of diarrhea among travelers to Guadalajara and perhaps other areas of Mexico.