The reproductive biology of marine pulmonate Onchidium sp., such as the copulation and spawning season, reproductive mode, fecundity, hatching and development of fertilized eggs, the effects of water temperature and salinity on embryonic development, the diapause of embryonic development, larval development and premetamophosis etc., were systematically investigated through either laboratory experiment, or field observation in both mollusk hatchery and culture ponds from 2004 to 2005. The results indicated that the marine pulmonate Onchidium sp., was protandrous hemaphrodite, and copulated with one another although the animal was often a functional male before the female organs developed. The main reproductive stocks were thos e of over 10 g in body weight while the smallest individual was the one with 3.1 cm long and 3.5 g in body weight. The reproductive season was usually during a period from the mid May to late September with the water temperature above 22 ℃. The period from June to August was the mating fastigium. The fertilized eggs were laid out in 15 days after courtship and mating, which usually occurred at night in 3 to 5 days during a high tide to low tide. There were often 6 obvious peaks of egg laying. Each egg mass usually contained 44 000 to 80 000 zygotes with a surface area of 15 to 30 cm². The hatching rate of the egg mass were insignificantly different no matter the egg mass were kept in water or in the moist air. It would take 10 to 14 days for hatching under the temperature between 26 ℃ and 35 ℃. The embryo ceased development under 23 ℃. The suitable salinities for embryonic development were between 6 ppt and 20 ppt. The veliger began to feed on microalgae 2-3 days after hatching. The free swimming veliger developed its periostracum of the body after a 23 day pelagic period. There might exist a process for the postmetamorphic larvae to periodically shed a cuticlelike structure from the dorsal body surface. More work still needs to be done to ascertain the condition and process of metamorphosis for pelagic veliger.