The metapopulation dynamics of Lernaeocera lusci and L. branchialis on sole Solea solea and flounder Pleuronectes flesus were studied in the Dutch coastal area. Both fish species harboured large numbers of parasites when they arrived in the coastal area in the spring. Between April and June all parasites detached from the intermediate hosts and infected the definitive hosts (0+ whiting Merlangius merlangus for L. branchialis, and possibly sand goby Pomatoschistus minutus for L. lusci). Thereafter, flounder remained almost parasite-free until autumn. This suggests that L. branchialis has only 1 generation per year. However, soles were infested again with L. lusci (in June and July), which detached to infest 0+ bib Trisopterus luscus, the typical definitive host for this parasite species. Thus, it appears that L. lusci has 2 generations per year. The flounder length and the infection intensity of L. branchialis were not correlated throughout the study period. Significant positive correlations were found between the sole length and infection intensity of L. lusci in late spring, but not in the summer or autumn. Throughout the year, both L. lusci and L. branchialis were aggregated within their intermediate host populations (variance much greater than much greater than abundance).