Sargassum horneri (Turner) C. Agardh (Sargassaceae, Phaeophyta) is commonly distributed on the Pacific and Japan Sea coasts of Honshu, mainland of Japan, and on the western and northern coasts of Kyushu. It grows subtidally, forming Sargassum beds which are called ‘garamoba’ in Japanese. These beds are known to be an important nursery ground for fishes, molluscs, crabs, and other animals. Initial germlings from eggs of the species appear in August or early September; through winter they reach their maximum growth and mature the following May. The mature plants discharge a vast number of eggs from their receptacles within the Sargassum bed. In addition to eating epiphytic algae and attached animals on Sargassum, animal predators within the bed may also consume eggs of Sargassum. Since the survival of this species is important to the ecology of nearshore areas, it is necessary to know the number of eggs of Sargassum horneri that are liberated per plant in order to assess the ecological balance