This study examines the barnacle symbionts on 168 blue crabs, Callinectes sapidus, taken from both shallow and deep estuarine environments in the area of Beaufort, North Carolina. The purpose of the study was to quantify the prevalence, intensity, abundance, and spatial distribution of the ectosymbiotic barnacle Chelonibia patula on blue crabs. The proportion of blue crabs fouled was 67%. There was no difference in the prevalence of barnacles on crabs from the shallow versus the deep environment. Results indicate female crabs were significantly more fouled than males. This suggests that the prevalence and intensity of barnacles are dominantly controlled by the migratory habits of the host, since female crabs spend more time in deeper waters of higher salinity, where they are more likely to be fouled by barnacle larvae. The spatial distribution of barnacles on the crab carapaces was controlled by the surface topography of the carapace with more barnacles on the lateral regions than medial. The orientation of the carinal to rostral axes of the barnacles on the carapaces of the host crabs was measured, but no preferred orientation was found. The costs and benefits of epibiosis are reviewed and the barnacle/blue crab relationship appears to be more beneficial to the barnacles than to the host blue crabs.