Microsatellites were used to determine paternity of Metacarcinus magister (Dana 1852), the Dungeness crab, embryos produced through controlled laboratory matings. Additionally, spermathecal and bursal sperm from mated females were genotyped to elucidate patterns of sperm storage and use. Spermathecal and bursal contents and egg clutches from female M. magister of unknown mating history were similarly analyzed. Genotyping of embryos revealed that bursal sperm are not used in fertilization. Multiple paternity was discovered in clutches from controlled matings and in 40% of clutches carried by crabs of unknown mating history. Males achieve last male sperm precedence through stratification of ejaculates within the spermatheca, but males may actually reduce their reproductive success by depositing too much sperm in the spermatheca. Stratification occasionally fails if the fresh ejaculate is large in volume, or when large volumes of stored sperm are present in the spermatheca, resulting in displacement of stored sperm towards the oviduct and multiple paternity within clutches. Sperm competition is interannual between the single primary male mate at each molt (adult females molt and mate once a year and sperm is retained across molts), and clutches fertilized with sperm as old as 2.5 years can develop to maturity. Female crabs may copulate for reasons other than to gain sperm for fertilization. Despite possessing internal sperm storage organs where oöcytes first encounter sperm, the act of fertilization in M. magister may occur externally.