Abstract
Among the three monogeneans which are found on the seabass Dicentrarchus labrax, Diplectanum aequans proved many times to be the most pathogenic in aquaculture. Aspects of demographical patterns peculiar to the parasite were analysed on hosts reared in floating cages, raceways and experimental tanks. While regulating mechanisms occur in the cages which control the demographic expansion, the increase of populations in the tanks sometimes becomes so high that it leads to heavy mortalities of D. labrax. By experimental colonization of the host, it was possible to confirm the aggregative form of the monogenean distribution in the fish-farmed stocks. From these epidemiological data, prophylactic measures are proposed.