Lipids of Neocalanus cristatus and Eucalanus bungii (C3 to adults), collected in March, May, and December from various depths (0-2000 m) were studied in the Oyashio region, western North Pacific. Total lipid and wax ester contents of younger N. cristatus stages increased during the development, being higher in May than in March and December. Major fatty acids of younger N. cristatus were 16:0, 20:5(n. -. 3), and 22:6(n. -. 3) and the dominant alcohols were 16:0, 16:1(n. -. 7), 20:1(n. -. 9)/(n. -. 11) and 22:1(n. -. 11). The energy-rich 20:1 and 22:1 moieties increased from the younger to the adult stages showing the importance of lipid biosynthesis which may be advantageous for successful overwintering and reproduction at depth. The 16:4(n. -. 1) fatty acid, characteristic of a diatom diet increased in May, particularly in the younger stages. Our results suggest that the diatom-dominated feeding mode of younger N. cristatus during the spring bloom is important for an effective accumulation of wax esters. In contrast to N. cristatus, E. bungii accumulated substantial amounts of triacylglycerols. The total lipid and triacylglycerol content increased slightly toward the older developmental stages. The major fatty acids were 16:0, 16:1(n. -. 7), 18:1(n. -. 9) and (n. -. 7), and 20:5(n. -. 3). There was no evidence of developmental or seasonal changes in the fatty acid composition. The differences in the lipid storage modes of both copepods via wax esters or triacylglycerols are species-specific but their fatty acid compositions varied according to diet and developmental stage, especially in N. cristatus. These lipid characteristics are discussed in relation to reproduction, feeding modes, diapause and overwintering strategies.