Trimethoprim and sulfamethoxazole were tested individually and in combination against El Tor and classical biotype strains of Vibrio cholerae. Determinations of the minimal inhibitory concentration in liquid media and by an agardilution method showed that classical strains were uniformly more resistant to sulfamethoxazole than were El Tor strains. In agar-dilution studies, trimethoprim was equally active against both biotypes. Combination of the agents produced a synergistic action against all strains tested. Testing for susceptibility to sulfonamide is suggested as a possible method for differentiating between El Tor and classical biotypes of V. cholerae.