The observation of 1,194 finless porpoises made through 11,549 km of sighting cruises in the Inland Sea and some ancillary observations provided the following informations. The porpoise migrates annually to and from the Pacific coast mainly through the two passes at the eastern Inland Sea. The highest density (4,900 individuals) is found in April at the beginning of parturition season, and the lowest (1,600) in early winter. The ratios of solitary adults, of solitary calves, and of cows with calf show seasonal fluctuation, and allow the analyses of the seasonality of reproduction. Gestation last for about 11 months. Parturition occurs between April and August with a possible peak in April and May. Most of the cows with calf born in April and May are suspected to leave the Inland Sea in summer months. Weaning occurs from December to June after a nursing period ranging from 6 to 15 months. Pregnancies in two contiguous years are not frequent. A 2-year breeding cycle seems to be most common. Annual production of about 870 calves (both sexes), and gross annual reproductive rate of 17.9% are provisionally estimated for the individuals in the Inland Sea.