Only few ecological and evolutionary relationships are as close as the one between parasites and their hosts. An interesting evolutionary suestion is to what extent host-parasite interactions can be considered as examples of co-evolution. We look for an answer to this question checking how the genetic variation of the parasite can be linked to the genetic variation of the host. This is done from a micro-evolutionary view, looking for 'recent' genetic variation on population level. Model organisms are the three-spined stickleback (Gasterosteus aculeatus, Teleostei) and its very host-specific parasite, the flatworm Gyrodactylus spp. (Monogenea).