We attempted to separate the additive variance of life-history parameters (size at first reproduction (SFR), proportion of total production allocated to reproduction) of Daphnia hyalina in a mesotrophic lake during different seasons into genetic components and phenotypic plasticity. Every month we randomly isolated juveniles of 20 clones from the lake and measured their life-history parameters immediately after isolation, i.e., under the influence of all modifying factors in the lake. The measurements were repeated after the clones had been kept in the laboratory for at least six generations without predator signals (controls) and with the addition of fish and Chaoborus kairomones. SFR increased monotonously from July to December in fresh field isolates. Laboratory controls showed always much larger SFR, but approached field value in December. Most of the size difference was due to maternal effects. Reproductive allocation showed a different pattern than SFR with a minimum in September both in field samples and laboratory controls. Fish kairomone reduced SFR in July and August when fish predation is high, but not later in the year. On the contrary, Chaoborus kairomone increased SFR in September and October when Chaoborus larvae are abundant in the lake, but not in summer. This indicates a seasonal shift in the clonal composition of D. hyalina populations towards clones adapted to the specific predation pressure. Reproductive allocation changed in response to fish kairomone in July and August, but not in response to Chaoborus kairomone. In this mesotrophic lake, fish have a stronger direct and indirect impact on Daphnia life history than Chaoborus larvae.