The triplefin, Enneapterygius etheostomus, is as mall reef fish attaining only 5 cm in total length. The fish is usually found solitarily on the shallow rocky reefs of southern Japan. Reproductive behavior of the fish was studied using SCUBA at Tsutsumi Island, Tsuyazaki (130'29'E, 33'47'N), Munakata, Fukuoka, during the period from July, 1985 through August, 1986. During the period, 175 specimens were collected with hand net and their gonadosomatic indices were obtained to know the spawning season. Digestive tract content of these specimens were examined and it was found that many conspecific eggs occur in the male digestive tract. The ratio of males which had eaten conspecific eggs and weight ratio of conspecific eggs to the total digestive tract content were obtained. The spawning season of the fish was considered to be from June through August from the gonadosomatic index analysis, but it was also pointed out that a part of the population reproduces in May and September. During the spawing season, the male exhibiting typical nuptial body color stayed within a very small territory of 0.2 m^2 for more than two weeks Maturation of the females which had home ranges around the male territory was not synchronous, but a small proportion of the female population matured almost everyday. Male courtship behaviors included figure 8 swimming and circling. Most of the spawnings were done by pair of a male and a female, but multi-female spawnings were also observed. Collection of eggs from the male territory showed that the male is guarding two or three clutches at the same time and it also showed that the adhesive eggs which take relatively long time to hatch almost always present in the male territory. From the digestive tracts of the male specimens, many conspecific eggs were found. The ratio of males which had eaten eggs averaged 37.9% of the specimens. The weight ratio of conspecific eggs to gut content attained 19.6% in wet weight. On the contrary, among 38 female specimens, only one individual was found to have eaten eggs. The gut content analysis showed that the males collected in the spawning season had eaten less than females collected in the same season and males collected before the spawning season. It was suggested that the males of E. etheostomus are eating their own eggs as energy source for egg-guarding during the spawing season.