Scanning electron microscopy (SEM) was used to observe the nature of association of mutant strains of Vibrio cholerae to infant mouse intestinal segments. Parental strain Inaba CA 401 was compared to the following mutant phenotypes: nonchemotactic, nonchemotactic reduced motility, flagellated nonmotile, nonflagellated nonmotile, and mucin nonutilizing. The virulence of the different strains was determined by oral-intra-gastric inoculation of seven-day-old mice. Localization of the parental strain during the course of infection in the patent gut of infant mice was followed by the fluorescent antibody (FA) technique. The highly virulent parental strain rapidly penetrated intervillus spaces in the upper and lower regions of the small intestine. By SEM the organism also associated markedly with the mucosal surface of intestinal segments. However, the vibrios were associated predominately with the intestinal mucin rather than the epithelial cell surfaces of villi. Nonchemotactic vibrios retained full virulence and ability to associate with the mucosa. The nonchemotactic strain with a reduced rate of motility was partially deficient in these properties. Nonmotile vibrios of either type were markedly reduced in both properties. The mucin nonutilizing strain associated normally but was weakly virulent. Low virulence of this strain may be due to a reduced ability of the organisms to multiply in the mucin.