Germ-free mice inoculated with Vibrio cholerae became colonized with vibrios throughout the gastrointestinal tract, but they did not become ill. High vibrio concentrations in the feces were observed throughout the 3 months of observation in spite of the presence of serum antibody. Reciprocal conversions of both Inaba and Ogawa serotypes occurred regularly after inoculation and could be correlated temporally with the appearance of serum-agglutinating antibody. Both of these smooth serotype were later progressively replaced by rough vibrios. Rough to smooth serotype reversions were also found in mice inoculated with a rough vibrio strain. Studies employing immunosuppression and selective vaccination suggested that such serotypic changes were a result of the selective effect of antibody within the intestinal lumen.