The aim of this project is to provide insight in the ecology and evolution of host-parasite interactions in natural populations. More specifically, we focus on the interaction between zooplank- ton (Daphnia sp.) and its microparasites. We investigate in which way the genetic structure of the host population affects the abundance of the parasites and their impact on the host popula- tion. We study whether the infection intensity is genotype speci- fic and whether differences in spore load and virulence have a genetic component. We want to verify the hypothesis of local and temporal adaptation between the host and the parasite. We cary out a field survey in three pons of 'De Oude Meren' (Heverlee). In an allozyme electrophoresis the Daphnia, investigate on the presence of will be analysed. The questions will be further exa- mined by experiments with Daphnia, isolated from different time points and out of the ponds and hatching traps.