Insertion mutations in two Vibrio cholerae genes, cya and crp, which encode adenylate cyclase and the cyclic AMP (cAMP) receptor protein (CRP), respectively, derepressed the expression of a chromosomal cholera toxin (CT) promoter-lacZ fusion at the nonpermissive temperature of 37 degrees C. In the classical biotype strain O395, the crp mutation increased the production of both CT and toxin-coregulated pilus (TCP) in vitro under a variety of growth conditions not normally permissive for their expression. The most dramatic increase in CT and TCP was observed with the crp mutant in Luria-Bertani (LB) medium pH 8.5, at 30 degrees C. El Tor biotype strains differ from classical strains in that they do not produce CT or TCP when grown in LB media. Incorporation of the crp mutation into El Tor strain C6706 permitted production of these proteins in LB medium pH 6.5, at 30 degrees C. In the infant mouse cholera model, the crp mutation decreased colonization in both biotypes at least 100-fold relative to the wild-type strains. The data presented here suggest a model whereby cAMP-CRP negatively regulates the expression of CT and TCP in both classical and El Tor biotypes under certain environmental conditions and also influences pathogenesis by regulating other processes necessary for optimal growth in vivo.