Two species of blue crabs were investigated with regard to infestation of the branchial chambers by the pedunculate cirriped Octolasmis lowei. In Callinectes danae, the frequency of infestation was 22.4%, and the mean intensity of infestation was 2.2 ± 0.2 (range: 1–8). Prevalence and mean intensity of infestation did not differ significantly between males and females of C. danae. Significant differences in prevalence were observed between juveniles and adults, between ovigerous and nonovigerous adult females, and among blue crabs in different molt stages. No correlation was observed between the prevalence of infestation by O. lowei and the carapace width of male or female blue crabs. In Callinectes ornatus, the prevalence of infestation by O. lowei was 12.1%, and the mean intensity observed was 4.0 ± 1.3 (range: 1–49). Prevalence of infestation did not differ significantly among crabs in different molt stages but differed between juveniles and adults and between male and female crabs. A positive correlation was observed between the percentage of infestation by O. lowei and the carapace width of C. ornatus males.