The zooplankton genus Daphnia is used as a model organism in ecological, ecotoxicological and evolutionary research. It is often used in experimental laboratory setups to examine life history traits under a variety of factors inducing stress. One type of stress, handling stress because of manipulation during the experiments, however, is often neglected. In a standard life history experiment, we found that several kinds of experimental manipulation may have a significant impact on the life history response of test animals. Especially renewal of the medium may reduce the reproductive output of the individuals significantly. In every experiment where the reproduction of Daphnia is an important trait under study, the negative effects of medium renewal and handling stress should be considered and care should be taken to keep handling of the individuals to a minimum.