1. This paper describes the effects of differences in food quantity and quality on selected life history traits ofCeriodaphnia pulchella. Animals were fed with four concentrations of two green algae,Scenedesmus obliquusandChlamydomonas globosa, given separately as well as in a 1 : 1 mixture.2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase,r, increased with food concentration. Life history parameters of animals raised onS. obliquuswere lowest, whereas no significant differences were found between animals fed withC. globosaand with the mixture of the two algal species.3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics ofC. pulchellain Lake Tjeukemeer showed that the effect of changes in food conditions during the growing season of this species was limited, although overall food levels seemed to be poor throughout the year.4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predatorLeptodora kindtii.