In recent years there has been insufficient study of longtail tuna, despite large harvests being recorded in the Sea of Japan. In the present study, therefore, we examine the relationship between longtail tuna catches and water temperature in the Sea ofJapan off the coast of Yamaguchi Prefecture. The results are as follows : I . The fishing season is from June to October. ２. We found from the study of juveniles caught by surface-water trawlling and oceanographic observation that : -Fork length ranged from 125 mm to 166 mm at the survey point. -Water ternperature was approximately 24℃ at the depth of the sampling position of longtail tuna juveniles. ３. Horizontal distribution of sea surface water temperature in 1996 was analyzed as follows using the NOAA satellite : -In July and September, adult longtail tuna were caught in abundance off the Futaoi Island, and the sea surface water temperature was as high as 24℃ around the set net position. -Temperature in August was above 24℃. -Temperature in October was below 24℃.