Exploitation rates for legal-sized (> 159-mm carapace width) male Dungeness crabs, Cancer magister, often reach 90% in the northern California fishery, thus making large male crabs scarce and possibly reducing mating success particularly among large (> 140-mm carapace width) female crabs. Relying on known laboratory molting and presumed mating, we characterized temporal changes in proximal sperm plugs of female C. magister from four days through 323 days following molting and mating. Although sperm plug surface area decreased steadily with increased days following mating, we found that complete sperm plugs and/or sperm plug remnants were present through approximately 180 days post mating, a period exceeding the approximately four month duration of the mating season for Dungeness crabs in northern California. Based on dissection and identification of sperm plug presence, we assessed mating success of female Dungeness crabs at two locations in northern California during June and July 1997, following the February-May mating season. Among 590 female crabs (87–167-mm carapace width) classified as having definitely molted on the basis of shell condition observations, we found that 93% had complete sperm plugs or remnants of sperm plugs in vaginal tracts. Among the 336 large (> 140 mm) definitely molted female crabs, sperm plug presence was 98.5%. Thus, despite the intense fishery on large male Dungeness crabs, sperm plug data provide evidence of essentially 100% mating success among large female crabs that molted during 1997.