Roach (n = 81) caught on 23 May, 1992 from oligotrophic, unpolluted Lake Peurunka were kept in cages over a 2-week period before moving half of the fish to nearby Lake Vatia, which is influenced by pulp mill effluents. Before moving the fish gill parasites were examined from 9 fish; 5 Dactylogyrus species, Gyrodactylus sp. and Paradiplozoon homoion were found, the main components of the infracommunities being dactylogyrids. Afterwards, 5 fish from each lake were studied weekly between 1 July and 17 August. Metazoan parasites were recorded from gill arches divided into 4 sections along the dorso-ventral axis. D. crucifer, D. nanus, D. micracanthus and D. suecicus occurred in both lakes throughout the study. Gyrodactylus sp., Ergasilus briani and P. homoion occurred in low numbers in both lakes. Differences between the lakes were seen in the prevalence of D. micracanthus which was higher in Lake Vatia and significantly increased abundances of D. crucifer, D. nanus and D. micracanthus in Lake Vatia. Abundances of other Dactylogyrus species remained, on average, at the same level in both lakes. No change during the experiment was recorded in the location of the two most common species, D. crucifer and D. nanus, on the host gills in either of the lakes, both species favouring the 2nd and 3rd gill arches and especially the inner parts of the gills. The apparent lack of competition between the 2 most common Dactylogyrus species indicates that no limitation of resources on roach gills occurred. This was also confirmed by the increased overlapping indices with increasing abundances between the two most common species in Lake Vatia.