Spotted seals (Phoca largha) and related crossbreeds maintained at Kamogawa Sea World breed seasonally from the end of January to middle of March. For contraception in these animals, the effect of a single administration of a contraceptive synthetic luteal hormone for dogs, proligestone (PRG), was investigated. The animals tested were 10 seals aged 4-24 years old, and a total of 35 trials were performed over five years. PRG was administered in 23 trials during January, which was one month before the estimated estrus, and in 12 trials during December of the previous year, which was two months before the estimated estrus. The dose of PRG was 5 mg/kg in 32 trials and 10 mg/kg in 3 trials. The effect of the contraception was judged by the presence or absence of delivery. Among 23 animals treated in January, 2 animals treated with 5 mg/kg PRG became pregnant, but the contraception was successful in the other 21 animals. Contraception was successful in all 12 trials treated with 5 mg/kg PRG in December. Overall, contraception was successful in 94.3% (33/35). Therefore, a single administration of 5 mg/kg PRG in December may be an effective method of contraception for seals.