A study of the Blue starfish Linckia laevigata in the Banda Islands, Moluccas, Indonesia, revealed 62% infestation by the molluscan prosobranch parasite Thyca crystallina. Infestation rate varies directly with the degree of water movement. A marked difference in settlement pattern according to parasite size is observed. The smallest limpets tend to settle on the upper surface of the distal parts of the arms and are randomly oriented. The largest parasites are found on the oral arm surface on the right hand side of the ambulacral groove and facing the starfish mouth. All of the attached parasites were female. Most of the large females had a dwarf male, one tenth the length, attached, under the mantle, at the anterior end. Amongst the small limpets, less than half had made a proboscis hole in the host integument and none had made an attachment disc lesion; of the large parasites 90% had the proboscis embedded in the host and 80% left an attachment disc lesion on the host. Small parasites can move very slowly over the host surface. Larger limpets are probably permanently attached. A detached parasite of any size is probably unable to reattach. There is evidence of an extended parasite breeding period and repetitive settlement on the hosts, which could cause the observed overdispersion without invoking a mature female parasite pheromone.