Summary.This paper deals with the relationships of two Jamaican crabs with their sacculinid parasites;Geograpsus lividus(Milne‐Edwards) infected bySacculina hartnolliBoschma, andMicrophrys bicornutus(Latreille) infected bySacculina bicuspidataBoschma. The following are the more important features discussed.1The level of infection and the incidence of multiple parasitism.2The modification of the host by the parasite.3The causes of this modification.A number of conclusions were reached.1In these two saceulinids multiple parasitism is due mainly to the invasion of the host by several larvae. In some other saceulinids, however, polyembryony is the principal cause.2The observed host modifications are similar to those described in previous investigations. They tend to change both male and female hosts into ovigerous females, both as regards morphology and behaviour.3The parasite affects its host by interfering with the endocrine system.4The modification of the host is of advantage to the parasite. This could explain why the sacculinid has evolved the power to induce these modification