To obtain a better understanding of the evolutionary interactions between host and parasite, we use as a model gill parasites be- longing to the genus Gyrodactylus and living on gobies from the genus Pomatoschistus Gill, 1864. Monogenea show a direct life cycle and display a strong host specificity, thus forming an ideal model for testing coevolution. An inventory of Gyrodactylus parasiting Pomatoschistus gobies in the North Sea shows a strong tendency towards phylogenetic host specificity. Three new species have been found, one described as Gyrodactylus longidactylus Geets, Malmberg 1998. However, identification based on morphome- try alone is difficult because of intraspecific variations indu- ced by climate and habitat. Hence, a first important contribution of genetic analysis lies in further determining the phylogeny of Gyrodactylus and defining the interspecific evolutionary rela- tionships and relatedness. .....