The reproductive biology and glycogen content of a subcrenated ark shell Scapharca kagoshimensis (Tokunaga, 1906) in Ariake Bay were investigated from April 2004 to May 2005 in relation to environmental factors such as water temperature, salinity, and plankton settled volume. In the male, onset of gamete development was observed in April. The period of mature testis and partial spawning was from July to September, and the spent stage was first found in September. In the female, gamete development was first observed in April, and continued until July. The matured ovary was seen in July, and partial spawning occurred in August, and the spent stage appeared from August. The gonads could not be distinguished in both sexes from December to March. Spawning in both sexes occurred during the period of highest water temperature (>25°C). The condition factor index remained high (>28.0) from April to June and rapidly declined to 18.9 during June and July. Glycogen content gradually decreased from 43.1 mg/g to 20.1 mg/g from April to July 2004 and gradually increased from 34.1 mg/g in January to 62.6 mg/g in April 2005. These results suggested that S. kagoshimensis in Ariake Bay was characterized by a spawning period in summer, monocyclic gametogenesis throughout the year, and an inverse relationship between gonad development and glycogen content.