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 enterotoxigenic Escherichia coli isolates nor among 89 Shigella strains to aztreonam, norfloxacin, ciprofloxacin, gentamicin, or furazolidone. High level (greater than 1000 micrograms/ml) resistance to trimethoprim/sulfamethoxazole (TMP/SMX) was found in 7% of E. coli and 3% of Shigella strains. Susceptibility patterns of 27 E. coli strains 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 Gudalajara and perhaps other areas of Mexico.