The paper reviews the biology of Contracaecum osculatum sensu lato and C. osculatum A, a member of the C. osculatum complex, parasitic in marine finfishes as larvae and in pinnipeds as adults in Japanese waters. Before C. osculatum A was discovered in 1998, larval nematodes identifiable as C. osculatum s. l. had been reported as various types of Contracaecum or C. osculatum. To date, larval and adult C. osculatum s. l. have been recorded from 13 species of teleosts and six species of pinnipeds, respectively, and there is a record of larval C. osculatum s. l. from euphausids. Larval and adult C. osculatum A are known from Alaska pollock Theragra chalcogramma and bearded seals Erignathus barbatus, respectively. Larval C. osculatum s. l. are commonly found in commercially important gadid fishes, especially in Alaska pollock. Two cases of human infection with C. osculatum s. l. have been reported in Hokkaido. Since larval C. osculatum s. l. can invade the intestinal mucosa of experimentally infected puppies, the larvae have the potential to penetrate the wall of the human digestive canal.