Seasonal changes of the two dominant Zooplankton species in Lake Trasimeno,Bosmina longirostrisandDaphnia galeata,was studied to evaluate factors regulating plankton production and the trophic state of the lake. Weekly samples were taken from January to December 1989. Parameters of population structure (total density, and densities of eggs, juveniles, and adults) and changes in body size were studied in relation to some environmental factors. Multiple regression models were used to analyze density and body size variations to evaluate the effects of temperature and food on growth of the two populations. The statistical models showed a significant influence of temperature and food on the seasonal patterns of density and body size of both species. In comparison with previous years, in 1989 a sharp decrease in the average body size ofD. Galeatawas observed; this was determined primarily by the scarcity of food, rather than by predation. On the other hand,B. longirostrisincreased in size, being favoured by its feeding flexibility and by the composition of phytoplankton, which was made up mainly of small‐sized algae;Bosminathus became numerically dominant overDaphnia.