Abstract The lipid content of Crangon crangon, Gammarus salinus, Neomysis integer, Praunus flexuosus and Orchestia gammarella collected from the reservoir of the Oldbury power station on the Severn Estuary ranged from 25 to 33% of the dry weight over the course of one year while the corresponding values for ash averaged 15–24%. Water accounted for 75–80% of the wet weight except in the case of O. gammarella (70%). The unusually high lipid values were probably connected with the highly favourable environmental conditions in the reservoir while both the size and reproductive condition of the animals were also important in determining the size of the lipid store. Phospholipids accounted for as much as 33% of the total lipid in C. crangon and for as little as 24% in N. integer and P. flexuosus. The predominant fatty acids in both the neutral and phospholipids of all species were 16:0, 18:1, 20:5 and 22:6.