Cyprid larvae of the lepadomorph Octolasmis colonize the gill chambers of the edible mangrove crab Scylla seratta (Forskål, 1755). In a natural population of 856 mangrove crabs from southern Thailand, 260 individuals were infested by 3,670 Octolasmis cor and 1,758 O. angulata, including 1,014 subadults, 168 cyprids, and 38 peduncles of the two species. This population of gill chamber symbionts was examined to investigate the relationship between barnacle size and barnacle spatial distributions. The habitat of the branchial chamber was partitioned for study into gills one through eight, the inside (hypobranchial) and outside (hyperbranchial) gill surfaces, and the proximal, medial, and distal regions of each gill. The collective data from 260 crabs were pooled for an analysis that showed a nonrandom relationship between the size of octolasmids and thier location within the gill chamber. On the inside gill surfaces O. angulata attained its largest average size on gills 3, 7, and 8, whereas on the outside surface the barnacles were largest on gills 4 and 5. Octolasmis cor attained its greatest average size on the inside surface of gill number 6. Comparisons of barnacles from the three gill regions also revealed some significant differences in average barnacle size. Positive correlations among barnacle size, barnacle number, and barnacle density were present. Moreover, there was a significant correlation between the total numbers of barnacles and the average size of O. cor on the inside surfaces of gill numbers 1 to 8, whereas it was not significant for O. angulata. There was also a significant correlation between barnacle densities and barnacle size in O. cor but not O. angulata. Positive correlations were also observed among higher numbers of barnacles, larger barnacles, and higher numbers of advanced reproductive stages. Areas with higher densities also were areas of higher average fecundity.