The parrotfish, Scarus sordidus Forsskal is widely distributed in the Indo-Pacific region and is the most abundant representative of the family Scaridae in the coral reefs of the Ryukyu Islands, Japan. We studied sexuality and sex reversal, general behavior and spawning behavior of these fish at the island of Kuroshima, the Yaeyama Group, the Ryukyu Islands during the period from October, 1975, to February, 1978. Results obtained are summarized as follows. 1) These fish are strongly dichromatic, with dark brown color phase (initial phase) and pale blue color phase (terminal phase). Initial phase fish consisted of females and males. Whereas, terminal phase fish were restricted to males. S. sordidus is a protogynous diandric species. All females are of initial phase, mature at about 130 mm SL or more. The color of females changes to terminal phase following sex reversal. Primary males mature at about 100 mm SL and change from initial phase to terminal phase at about 150 mm SL. Color change can be artificially induced by injecting testosterone. 2) These fish are omnivorous, and feed strictly on coral polyps and algae. In feeding, they form mixed foraging groups with other scarids, mullids and labrids. Such groups are often attacked by several pomacentrid fishes, which hold territories on coral substrata within the lagoon moat. Individuals within mixed foraging groups, however, can get food from the pomacentrids' territories by swamping. At night, these fish sleep within mucous envelops. 3) S. sordidus spawns from April to August at Kuroshima. Spawning begins immediately after high tide and continues for about one hour at reef channels or outer reef slopes. Both pair and group spawning were observed. Terminal males spawn in pairs with single females, whereas, initial males spawn in groups.