The potential of Vibrio cholerae O1 to attach to and colonize the carapaces of shrimp and crabs was evaluated. One million cells of V. cholerae O1 were spread within a circle on the external surfaces of separated carapaces and stored at 22 +/- 0.2 degrees C in a moist environment to permit adherence. Attached vibrios were counted directly by an immunofluorescence technique and by the pour plate technique after detachment of the cells. To study the colonization process, rifampicin-resistant strains of V. cholerae O1 were used. V. cholerae O1 strains, including those resistant to rifampicin, were able to attach to shrimp and crab carapaces. Dorsal crab carapaces showed higher levels of attachment than ventral carapaces. Colonization of V. cholerae O1 on these carapaces was also demonstrated. Both attachment and colonization on the shrimp exoskeleton were optimal at a salinity of 1.0 to 1.5%, a pH of 6.0 to 7.0, and a temperature of 37 degrees C. Less than 2% attachment at 3 degrees C contrasted with >20% attachment at 37 degrees C. Even at 3% NaCl, some attachment was observed. Although attachment percentages may appear low (2 to 20%), they represent significant numbers, about 3.7 to 5.6 log10 CFU per carapace. A rugose V. cholerae O1 strain attached to and colonized the shrimp carapace in a fashion very similar to that of the smooth strain from which it was derived. The ability of V. cholerae O1 to attach to and colonize exoskeletons of edible crustaceans provides a potential means of survival in aquatic environments. Concentrations of vibrios that may be reached on a single crab or shrimp carapace are clearly of concern with regard to public health.