The rhizocephalan barnacle Briarosaccas callosus causes sterilization and reduced growth of red Paralithodes camtschatica, blue P. platypus, and golden Lithodes aequispina king crabs, commercially important species in southeastern Alaska. The prevalence of the parasite (percentage of king crabs infected) was monitored from 1979 through 1985 within a 115-km radius of Juneau. Prevalences were typically less than 1% in commercial landings of legal red and golden king crabs (males only; minimum carapace width 179 mm), but reached 12% in landings of blue king crabs (males of 165 mm minimum carapace width). Annual surveys of commercial red king crab pots, which contain females and sublegal males, usually indicated prevalences of less than 1%, though 50% was reached in exceptional cases. On-board samples from commercial boats that included females and sublegal males confirmed low prevalences (<1%) for red king crabs, but 20% of golden king crabs and 76% of the blue king crabs were parasitized. Within broad temporal and geographic variation, blue king crabs were the most, and red king crabs were the least, heavily parasitized by barnacles of the three host species considered. Prevalences tended to be greater for smaller than for larger hosts, for females than for males among blue and red king crabs, and in turbid than in clear water.