In order to obtain data on the reproductive pattern of the clonal cnidarian Zoanthus sansibaricus, polyps were sampled by scuba gear at Taisho Lava Field, Sakurajima, for 24 months between April 2000 and March 2002 (polyps collected weekly for breeding season). According to cross-sections, Zoanthus polyps were divided into three sexual types; male, female and asexual, and were found in the same colony. At Sakurajima, Zoanthus sansibaricus spawned in the middle of July, releasing oocytes and sperm. These spawning events occur synchronously with moon phase. In gametogenesis of Zoanthus sansibaricus, oocytes became recognizable in February and grew rapidly from the end of June onward. Spermatocytes became recognizable in June and matured rapidly in the middle of July. After spawning events, oocytes still remaining in the endoderm were absorbed into Zoanthus tissue quickly.