Gudgeon (Gobio gobio; Cyprinidae) were collected monthly from the Vlara stream (Czech Republic) during 2004 and examined for the presence of the metazoan parasites. The locality is downstream of the Slavičín town and the military factory outlet.The monogenean ectoparasites Gyrodactylus spp., Dactylogyrus cryptomeres and Paradiplozoon homoion were abundant species. P. homoion is fish ectoparasites with a direct life cycle, where the adults live attached to fish gills using two haptors with four pairs of clamps and a pair of central hooks. There are no reports about mass death of fish host caused by this type of parasite. In our study, over 26% of P. homoion showed abnormalities, indicating either a stressful environment or that the quality of the fish host was suboptimal for parasite development. The abnormalities of diplozoids are categorized into seven types and are related to the attachment apparatus. There was seasonal effect on P. homoion abundance being the lowest during from April to July (from 2.4 to 3.0 parasites per host) and the highest in October (6.65 parasites per host). However, the proportion of parasites with abnormalities did not vary among seasons. It is assumed that abnormalities are mainly caused by environmental pollution.