Seasonal fluctuations in mean dry weight of the copepod Calanus helgolandicus were large and contrasted markedly with the small seasonal fluctuations of cephalothorax length. Dry weight was related linearly to cephalothorax length on logarithmic axes by a series of regression equations which were significantly different for each of 8 cruises. Carbon and nitrogen were likewise linearly related to dry weight on logarithmic axes and followed the seasonal fluctuations in dry weight. Dry weight and cephalothorax length showed little or no correlation with food concentration, ingestion rate, population numbers, population biomass or temperature. However, seasonal changes in dry weight could be explained in terms of Vidal's (1980a, b: Mar. Biol. 56: 111-134; 135-146) laboratory studies which relate growth rate and development time to body size, food concentration and temperature.