Parker's (1970a) hypothesis that the overlap of multiple mating sperm in the female's storage organs promotes sperm competition is tested here for the first time in Crustacea: specifically, the mechanisms and consequences of sperm competition are detailed for the spider crab Inachus phalangium. Females of this species store ejaculates from successive copulations with different males discretely and consecutively in sac-like twin seminal receptacles. During copulation males transfer a large quantity of a sperm-free seminal plasma, followed by the sperm which is stored in small spermatophores and forms a densely-packed sperm packet. It was shown, using 3H-thymidine-labelled ejaculate, that the last male to mate displaces the ejaculate of his predecessors dorsally into the apex of the receptacle. Sperm of previous matings are sealed in with the hardening seminal plasma (sperm gel) and are thus prevented from being used to fertilize eggs, while the last male to mate places his sperm closest to the oviduct and vaginal openings. In experiments using the ‘sterile-male’ method, sperm from the last male to mate gained all fertilizations in subsequent broods. The seminal plasma forms the sperm gel in ghost spider crabs which is used for displacement of previously stored sperm, whereas various other brachyuran taxa use seminal plasma to produce the sperm plug, which prevents a male's sperm from being displaced.