A deep-living, open-sea population of Monoporeia affinis from the northern Baltic Sea was studied for seasonal variations in the gross biochemical, lipid class and elemental composition during 1991 to 1993. The seasonal cycle in the composition of this benthic, deposit-feeding amphipod was largely determined by the brief period of spring phytoplankton bloom sedimentation and the longlasting deficiency of good-quality nutrition during late autumn-early spring. The level (% dry wt) of lipid was lowest in March-April (21 to 30%, range: interannual variation), increased rapidly in early summer (27 to 43%) and peaked in the autumn (38 to 44 %). Reciprocal to lipid, the level of protein was highest in the spring (19 to 29%) and lowest in the autumn (17 to 23%). Carbon and nitrogen levels followed closely the patterns of lipid and protein, respectively. Triacylglycerols were invariably the main lipid class (67 to 95% of total lipids), while phospholipids formed 4 to 23% and other classes <7 % each. Due to high variability in biochemical composition, the energetic value of the body matter of M. affinis showed great seasonal variation. Using body composition and previously determined metabolic rates, calculations showed distinct seasonal and life-cycle variability in the bioenergetic strategy of the amphipods. Interannual variability in the body composition of the amphipods was noted, especially between 1991 and the 2 follo.wing years. Previously recorded long-term oscillations in the abundance and biomass of M. affinis populations and the relation of these oscillations to pelagic events and sedimentation is discussed in the light of the present results.