Total lipid (TL) and lipid-class composition in Acanthostepheia malmgreni (Crustacea: Amphipoda) from 240 m depth in Conception Bay were determined from samples collected monthly throughout 1999 and 2000. Variation in lipids among life history stages was studied in relation to the reproductive cycle of A. malmgreni and the timing of the major annual sinking event following the spring phytoplankton bloom in April. Total lipid remained relatively constant in first year juveniles [mean 4·4% dry mass (DM)], after which it gradually increased with the first signs of development of sexual characteristics and the sedimentation of spring bloom material in May. Triacylglycerol (TAG) was the dominant neutral lipid in all stages, with phospholipid (PL) the dominant polar lipid. Highest net accumulation of TL and TAG occurred in a group of large age 2+ individuals from October 1998 to May 1999, and in age 1+ immature males and immature females from July to November 2000 (1·4 mg TL month−1; 0·8 mg TAG month−1). Phospholipid quantities within the population were relatively constant at ≈2% DM. Maximum TL in the population (13% DM) was observed in large adults and some large immature individuals, and significant decreases in both TL and PL were recorded in mature females during the brooding period. This study reveals a dependence of A. malmgreni on seasonal lipid accumulation for reproduction, and a clear trophic link between the amphipod population and the pelagic production cycle.