1. In 1974 the population dynamics and production of the crustacean plankton in eutrophic Lake Biel were investigated. Earlier papers concerning the crustacean fauna of Lake Biel enabled the succession to be followed qualitatively and partly quantitatively over the last 80 years. 2. The samples were collected between 0 and 70 m at the deepest station in the lake by means of a pump at intervals of 2 to 3 weeks. As the most important environmental factors, oxygen and temperature profiles were taken (fig. 4). In order to distinguish between the different copepodid stages of the various copepod species morphometric differentiating criteria were worked out (fig. 2 and 3). The calculation of crustacean production was made accroding to Mittelholzer [57]. The dry weight of single individuals was determined from the length measurements using the regression equations of Dumont et al. [7] (table 1). 3. The crustacean plankton was numerically dominated byCyclops vicinus lobosus (April/May), byDaphnia longispina hyalina (June/July) and byAcanthocyclops sp. (August/September) (fig. 16). The population maxima of these three species reached at times 1.2 million individuals per m2. In winterEudiaptomus gracilis dominated with a maximum of 100,000 individuals per m2. The density of the 4th copepodides ofC. vicinus lobosus in diapause (May 1974 to January 1975) was 600,000 individuals per m2 lake bottom (fig. 5). 4. The pelagic crustacean production in 1974 amounted to 26.2 g dry weight per m2. 81% of this was contributed by the three speciesD. longispina hyalina, C. vicinus lobosus andAcanthocyclops sp. the percentages of the total production for the individual species in 1974 were as follows:D. longispina hyalina 40.9%, C. vicinus lobosus 26.3%, Acanthocyclops sp. 13.7%,E. gracilis 5.9%, Bosmina sp. 5.1%,Mixodiaptomus laciniatus 2.4%, D. longispina galeata 2.4%, D. cucullata 1.3%, C. bohater 0.8%, C. abyssorum praealpinus 0.7%, Mesocyclops leuckarti 0.5%. This crustancean production output lies in the same order of magnitude as that of other eutrophic lakes. 5. Since 1900 the composition of the crustacean species has changed basically both in quality and quantity (table 7):