Abstract
The bass Dicentrarchus labrax is infected by two monogenean Monopisthocotylea : Diplectanum aequans and Diplectanum laubieri. The stages of their life-cycle were studied in order to estimate their growth rate and to identify the determining criteria for all ages. Their growth is almost synchronous ; the growth patterns are propounded. At about 20°C., one month is required, after their fixation on the host gills, for achieving the adult stage (egg-laying adult) of both species. The only two reliable criteria for distinguishing between the species are the squamodisc in young individuals, and the squamodisc and the penis in adults. When these structures are lacking in the youngest stages, the two species cannot be distinguished.