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In order to study sperm competition and mating dynamics in the snow crab Chionoecetes opilio, a genomic library was established with the goal of identifying highly polymorphic microsatellite markers. Six pairs of DNA primers were designed to amplify markers Cop3-4, Cop4-1, Cop5, Cop10, Cop24-3 and Cop111 by the polymerase chain reaction (PCR). All markers produced patterns as expected from single loci inherited in a mendelian fashion, except for Cop5 which revealed a multi-locus banding pattern. The cross-amplification of the six loci in seven additional crabs species revealed DNA polymorphisms at one or more loci for each species. Markers Cop3-4 and Cop24-3 were used to determine paternity of larvae of primiparous females both from the wild and from multiple mating experiments under laboratory settings. The two markers were also used to genotype the contents of female spermathecae in order to determine the number of number of male genotypes present. Spermathecal contents of wild-caught females were cut into several cross-sections and each section genotyped individually. Histological analysis of spermathecae was carried out to complement genetic data in order to elucidate patterns of sperm competition. Single paternity was observed for the progeny of all females. The analysis of laboratory females showed displacement was the mechanism by which single paternity was obtained by the last males to mate. The analysis of wild females revealed that their spermathecae contained on average the sperm of at least 3.7 males. Larvae appeared to be sired by males whose genotypes were found in the spermathecal cross-sections toward the blind-end of the spermathecae. This suggested that they were the first males to mate with females they guarded until oviposition, and females remated with other males thereafter. Also, a comprehensive account of the mating dynamics was carried out in a wild population of the Northwest Gulf of Saint Lawrence (Eastern Canada) and demonstrated the e