This paper deals with the spawning season and sex ration of ribbon fish Trichiurus lepturus in the western Warasa Bay from June, 1981 to February, 1983, which were caught by longline, trolling, set net, purse seine and Danish seine. It was found that the spawning season was from June to September and the highest value fo gonad index of both sexes were shown from July to August. In the early spawning season. maturation of gonad was found in large sized fish more than 300 mm of anal length, and maturation made progress in orde of size. Just before the spawning season, schools of larger fish with the sexratio 1 : 1appeared in this area. In the spawning season. females and males dominated the near shore and offshore, respectively. It was suggested that the difference in the distribution of the sexes was concerned with the mating system.