The processes of gametogenesis and maturation in broodstock of the scaly-type pen shell Atrina pectinata, in suspension culture were studied throughout the year. Gametogenesis in both sexes was initiated in February, when the water temperature was lowest. Gonad development and subsequent maturation occurred from March to September as the water temperature and food density increased. Spent gonads were found in October. Analysis of oocyte diameter frequencies in females just after spawning suggested that pen shell females spawn repeatedly during the breeding period. A number (1.6%) of hermaphroditic animals were also identified. Glycogen contents gradually decreased during the vitellogenic and mature stages. These results suggested that gonad development of pen shells in suspension culture was characterized by annual synchronicity in males and females, a spawning period from May to September resulting from monocyclic gametogenesis throughout the year, and an inverse relationship between gonad development and glycogen level. This study provides basic information applicable to mariculture of pen shells for commercial production.