Abstract – The seasonal variation in lipid class composition (triacylglycerols, free fatty acids, cholesterols and polar lipids) was described for juvenile Atlantic salmon sampled at 12 occasions over an 18-month period in the Norwegian River Alta (70°N). Lipid class composition was determined for pooled samples of groups of fish sorted by cohort and condition factor. Mass-specific lipid content ranged from 2.3% and 8.1% of fresh mass, and mass-specific triacylglycerol content between 0.16% and 5.6% of fresh mass. Total lipid content decreased by 34–57% during the winter period, before increasing by 155–176% between May and July. A large proportion of the variation in total lipid content was due to variation in triacylglycerol content (r2 = 0.95), except for variations at low total lipids levels when triacylglycerols were absent. The temporal variation in polar lipid content was, although to a lesser degree, related to total lipid content (r2 = 0.62), indicating that the fish also utilised polar lipids as an energy source during periods of starvation. Analyses of 39 individual parr collected in May showed a large individual variation in lipid class composition in the most critical period of the year. It is concluded that information on variation in lipid class composition may give more precise and detailed information of energy status during critical periods, although total lipid content will in most cases give a fair description of the energy status of the fish.