Declining population sizes reported for a number of marine species have emphasised the need for information on mating systems to be incorporated into conservation and management strategies, particularly for exploited species. The brown crab Cancer pagurus L. supports a major fishery; however, many aspects of the species’ mating system, such as paternity patterns, are unresolved. In this study, 3 microsatellite loci conferring a high degree of statistical power were used to determine whether broods from 18 ovigerous females collected from the English Channel had been fertilised by multiple males. Despite the capacity of this species for long-term storage of sperm and the suspected potential for females to use sperm from multiple males simultaneously, no evidence of multiple paternity was detected. Hypotheses as to the predominant processes leading to single paternity are discussed. In systems where females exhibit genetic monogamy the effective population size is constrained by the number of females. Such systems may be particularly susceptible to declines if females are removed. This has important implications for management of C. pagurus populations, as fishery data indicate that females are more heavily harvested than males.