The seasonal variation in lipids, and fatty acid composition of neutral lipid(NL) and phospholipid(PL) contained in the muscles of hibernant fish(loach) were investigated. In the investigation, four groups of wild loaches ere used, those caught May, 1993(before spawning period), August, 1993(after spawning period), November, 1993(just before hibernation) and March, 1994(just after hibernation). The NL was the most abundant in muscle lipid, followed by PL and glycolipid(GL), and also the composition and variation patterns were almost similar between female and male throughout all the periods. NL content began to increase gradually after March and came to the maximum in August(under 90%) ; hereafter got to decrease gradually again and came to the minimum in March of the next year(under 60%). While the variation-pattern of GL and PL was in inverse proportion with that of NL and their content came to the maximum in March of the next year. The NL and PL fractions were mainly consisted of triglyceride and phosphatidyl choline respectively, and both of the contents showed the highest value in August, the lowest in March of the next year. The major fatty acids in NL fraction were 16:0, 16:1, 18:1, 18:2 and 18:3($\omega$3). In particular, the ratio of 16:1 was much higher than that in PL, while those of highly unsaturated fatty acids(HUFAs) such as 20:4($\omega$6), 20:5($\omega$3) and 22:6($\omega$3) were much lower. The ratio of 16:0 in NL was almost constant through the year, however those of 16:1, 18:1, 18:2 and 18:3($\omega$3) showed a tendency to increase after March and were about 2~4% higher in May, compared with that in March, while those of HUFAs were about 2~4% lower. In particular, the ratio of 16:1 in NL continuously decreased after May and markedly decreased in March of the next year, but those of 18:1, 18:2 were almost constant until November. In August, saturates and monoenes were higher levels due to the slight increase of 18:0, 14:1 and 17:1 than in the other periods, while those of HUFAs decreased and the unsaturation(TUFA/TSFA) of NL showed the lowest value in August. On the contrary, saturates and monoenes gradually decreased after November and the most of them including 18:0, 16:1, 18:1 and 18:2 rapidly decreased in March of the next year, while 20:1, 20:4(~6), and 20:5(~3), and 22:6(~3) were in inverse proportion. PL consisted mainly of 16:0, 16:1, 18:1, 18:2, 20:4(~6), and 20:5(~3). Saturates and monoenes-composition showed almost no difference in March and May, while polyene acids ratio showed similar changes with that of NL. In August, the ratio of saturates(16:0, 18:0) were high but that of HUFAs were low, and after November 16:0, 18:0, 18:1, 18:2 and 18:3($\omega$3) began to decrease, while most of HUFAs increased in their composition-ratio until the marking increase of their unsaturation and the big changes of 18:2, 18:3($\omega$3), 20:4($\omega$6), 22:6($\omega$3) in March of the next year.