The role of cholera toxin (CT) in mucosal colonization by Vibrio cholerae O1 was studied in rabbits by using toxinogenic V. cholerae and nontoxinogenic (A-B+ or A-B-) recombinant mutants derived from them. After oral inoculation, toxinogenic strains colonized intestinal mucosa significantly more efficiently than did either A-B- or A-B+ mutants; average colonization was increased 1.5- to 30-fold with toxinogenic strains, depending on the inoculum used and the portion of intestine studied. Additionally, colonization by an A-B- mutant was increased to the levels of its toxinogenic parent by coadministration of CT with the inoculum. We conclude that CT contributes significantly to mucosal colonization by V. cholerae and that this effect is not due to an interaction of the CT B subunit with its mucosal receptor. The possibility that this effect contributes to the in vivo selection of hypertoxinogenic variants of V. cholerae is considered.