Seasonal development of gonads was studied in the hagfish, Eptatretus burgeri, caught near the Misaki Marine Biological Station facing the Sagami Bay of the Pacific coast of Japan, during periods from October 1971 to March 1973 and from November 1977 to December 1978. The hagfish were collected at water depths of 6 to 10 m (shallow water) in Koajiro Bay close to the Station or water depths of 50 and 100 m (deep water), about 1,600 and 4.800 m west to the Station. They were collected monthly from both shallow and deep waters, but they could not be collected from shallow water during warmer summer months (July-September) due to seasonal migration. The mean total lengths of the females collected in both shallow and deep waters during January to April were 40.7± 0.4 cm (± SEM; n=317) and 34.5± 0.3 cm (± SEM; n=556), respectively. In both locations, developing eggs longer than 5 mm were found in females larger than 39 cm in total length. There was no difference in the annual growth curves of developing eggs between shallow and deep water: the sizes of the developing eggs were the smallest in October, and was the largest in September. There was no apparent difference in testicular development between two locations, so these data were combined. Testicular development occurs in males larger than 38 cm in total length. The testis weight was the heaviest in July, and was the lightest in September. In autumn and winter, most testicular follicles contained only spermatogonia. In spring, follicles containing spermatocytes increased. Follicles with spermatids or maturing sperms were relatively abundant in summer. Although egg deposition is supposed to occur in September and/or early October somewhere in deep water, both testis weight and spermatogenesis were minimal at this time. Why testis development is lowest at the time when females presumably deposit eggs is discussed in relation to the question of the location of the spawning ground.