Aspects of the life cycle of the cumacean D. rathkei, the most important food item of demersal fishes in the Western Baltic Sea, were investigated from the Underwater Laboratory (UWL) 'Helgoland' in Lübeck Bay. In Aug the population density was 10.1 ±3.6 individuals 100 cm Super(-2) in fine-medium sand (Md = 292 µm), and 3.6 +- 2.4 individuals in coarser medium sand (Md = 470 µm). In both substrata, abundance values decreased until Oct, at each station to a different extent. Particular attention was paid to vertical migrations of D.rathkei. Plankton catches and numerous direct observations provided evidence that the pelagic phase, observed at night, is always combined with ecdysis. During short periods of high swarming andmoulting activities, the population structure changed considerably: juveniles of the 3rd stage dominated inearly Aug; thereafter they were replaced by 'prematures', the last juvenile stage. The first mature individualswere observed in Oct. Some data are given on growth rate.