Wild-type Vibrio cholerae of both El Tor and classical biotypes (strains N16961 and 395, respectively) and nonmotile mutant derivatives with and without flagellar structures were characterized in three different animal models: (i) the rabbit ileal loop, (ii) the removable intestinal tie adult rabbit diarrhea (RITARD) model, and (iii) the suckling mouse model. Both the wild-type strains and nonmotile mutants were toxinogenic in the rabbit ileal loop and the suckling mouse models. However, all of the nonmotile mutants produced significantly less fluid accumulation than did the wild-type parental strains. The two nonmotile mutants of strain N16961 did not adhere to rabbit ileal mucosa, but both nonmotile mutants derived from strain 395 exhibited adherence. In the RITARD model, the motile El Tor strains were more virulent than both the flagellate and aflagellate nonmotile mutants (all infected rabbits died within 18 h), while the nonmotile mutants, when fatalities occurred, required 78 to 105 h to produce a fatal outcome. Likewise, the motile classical parent 395 produced a fatal outcome within ca. 25 h, while nonmotile mutants required 69 to 96 h. The nonmotile flagellate strain KR31 was not significantly more virulent than the nonmotile aflagellate strain KR26. Of the two classical nonmotile mutants, KR1, which produces a coreless sheathlike structure, was clearly more virulent (5 of 10 rabbits died within 96 h), while KR3 (nonmotile, aflagellate) did not produce fatalities in any of the 10 rabbits tested. Similarly, no significant difference in diarrheagenicity or colonizing ability was detected between the two nonmotile mutants derived from the El Tor strain, but the classical nonmotile mutant with the coreless sheath caused significantly greater diarrhea and colonized for a longer time than did the isogenic nonmotile aflagellate strain, KR3. No significant differences between the nonmotile mutants were detected in competition studies done with suckling mice. Analysis of the wild-type and mutant strains in these three animal models clearly demonstrated a role for motility in V. cholerae pathogenicity, while analysis of only the nonmotile mutants derived from the classical parent suggested a role for flagellar structures.