The marine green algal genus Bryopsis has both monoecious and dioecious species. It is suggested that monoecious species might have evolved from a dioecious ancestor. Monoecious species produce male and female gametes on the same plant. Each gametophyte of dioecious species produces only male or female gametes. We approached the evolutionary origin of monoecy. Protoplasts were isolated from male and female gametophytes of a strictly dioecious strain of B. plumosa. They developed into coenocytic macrothalli. Macrothalli successfully produced swimming cells, which looked similar to biflagellated normal gametes even when protoplasts were mixed between male and female with male/female protoplasm ratios of 10:0; 9:1; 7:3; 5:5; 1:9; 0:10. Size of swimming cells depended on the male/female protoplasm ratio. Swimming cells showed normal phototactic behaviors of male and female gametes and resultant planozygotes (i.e. male gametes: non-phototactic; female gametes: positive phototactic; zygotes: negative phototxis). Negatively phototactic swimming cells were quadriflagellated with two nuclei as a result of fusion. But they never developed into microthalli. We concluded that natural monoecy observed in this genus did not originate from hybridization of protoplasm between the sexes, which might often occur in nature.