By examining 99 Vibrio cholerae O1 isolates from cholera epidemics in a variety of areas in Laos in 1998, we found two unusual characteristics of the organisms. All except 4 isolates were moderately resistant to tetracycline without having plasmids, and susceptibilities to the other drugs were as expected. Eleven isolates showed the same level of susceptibility to polymyxin B as classical V. cholerae O1. With the exception of 4 tetracycline-susceptible strains, all the isolates were resistant to or poorly sensitive to the vibriostatic agent O/129 (2, 4-diamino-6, 7-di-iso-propyl pteridine phosphate). All isolates, including those susceptible to polymyxin B, produced El Tor hemolysin which was neutralized by anti-El Tor hemolysin and were resistant to cholera phage IV, indicating that they were of the El Tor biotype.