Experimental exposure to bleached kraft pulp and paper mill effluent (BKME) at a concentration of 10% significantly reduced the abundance and mean number of species of Dactylogyrus on the gills of naturally infected roach, Rutilus rutilus, over a 3 week period. Seven Dactylogyrus species were recorded which differed widely in their susceptibility to the effluent. The experiment coincided with a natural spring peak in dactylogyrid infections on roach. BMKE exposure did not prevent parasite reproduction but post-larval abundance was significantly higher on control than effluent-exposed hosts. There was no evidence of a BKME-induced shift in microhabitat distribution of dactylogyrids. Elevated levels of infection with Dactylogyrus spp. have been recorded on roach from a Finnish lake containing relatively low concentrations of BMKE. The high exposure concentration in this experiment produced a similar reduction in dactylogyrid infections to that reported in separate studies in close proximity to Swedish pulp and paper mills. The discrepancy between the results of the Finnish and Swedish field investigations is therefore considered to be due to differences in BKME concentration between the study areas.