ABSTRACT Early in 1981 marine Crustacea (Decapoda) indigenous to Singapore waters were examined for epizoic pedunculate barnacles of the genus Octolasmis (Gray, 1825). Twenty-seven of the 56 decapod species examined were hosts of up to 7 Octolasmis species each. The Octolasmisspecies observed included O. angulata, O. bullata, O. cor, O. lowei, O. neptuni, O. tridens, O. warwickii, and three unnamed species. Of the 10 Octolasmis species one (rarely two) was found externally on the body and appendages and nine species were found internally in the gill chambers. Host specificity varied, but most Octolasmis species occurred on several host species. With one exception, significantly more barnacles were found on the inner side than on the outer side of the host gills. Barnacles were unevenly distributed over the proximal, medial, and distal portions of the gills. Generally barnacle species had distinctive distribution patterns over the gills. In the crab Scyllla serrata densities of two Octolasmisspecies were highly correlated, whereas in Charybdis cruciata the correlation between two other species of barnacles was less pronounced.