The maturity of the north Pacific giant octopus Octopus dofleini and its mating season were investigated by regular samplings during the period from November 1992 to November 1993. The sampling area was the coastal waters off the northern tip of Honshu (Cape Shiriya, Shimokita Peninsula, Aomori pref.). Females were classified into immature and mature stage by coloration of the ovary. In males, maturation stages were classified into three stages: mature stage by the length of spermatophore exceeding 70cm long; maturing stage by the length of spermatophore less than 70cm long; and immature stage by the absence of spermatophore. Mature females appeared from January to May. Male maturing specimens were present from March to December and mature ones appeared from December to May. Immature specimens of both sexes were taken throughout the year. Mated females were caught from January to April. The number of ovarian eggs ranged from 38, 000 to 94, 000.