The distribution of adult Pseudodactylogyrus anquillae and P. bini and postlarvae was recorded on glass-eels and pigmented eels of different size and infection intensity. The two species occupied different microhabitats on the host's gill apparatus but the preferred sites depended partly on host size. Postlarval migration was indicated. Mortality of young parasites, reinforcement of reproductive barriers and competition seems to be less likely explanations of the spatial distribution.