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Seaweed beds in Japanese coastal waters have significantly declined in recent years and feeding by herbivorous fish has been identified as one of the potential causes of this decline. In the western coastal areas of Kyushu, seaweed consumption by fish species such as the mottled spinefoot (Siganus fuscescens), sea chubs (Kyphosus spp.), and the Japanese parrotfish (Calotomus japonicus) has become a matter of concern. Our research group has been investigating the biology of herbivorous fish in the coastal waters around western Kyushu and Okinawa. This paper presents some of the results of our work with a focus on the biology of herbivorous fish, including their distribution, age, growth, sexual maturity, sex change, feeding, behavioral ecology, and population structure. Investigation of the stomach contents of herbivorous fish revealed that Kyphosus bigibbus fed mainly on the seaweed sargassum throughout the year. In contrast, other fish supplemented their diet of seaweed with amphipods and other organisms. Experiments to determine food preference were performed on captive K. bigibbus, and their results supported the results of the stomach content analysis. Both experiments showed that the fish selectively fed on Sargassum fusiforme and Undaria pinnatifida. Herbivorous fish off the west coast of Kyushu were tracked using a biotelemetry technique, which indicated that the fish inhabited seaweed beds during the daytime. The activity of S. fuscescens and K. bigibbus markedly declined when the water temperature decreased to approximately 20°C and 17°C, respectively, but it was observed that these fish overwintered in these areas. This study demonstrates that the recent rise in winter ocean temperatures has extended the period of activity of herbivorous fish. The results contradict the hypothesis that herbivorous fish species migrate southward during colder periods., Nagasaki University Major Research Project: Restoration of Marine Environment and Resources in East Asia, Coastal Environmental and Ecosystem Issues of the East China Sea ,pp.181-190; 2010