The mesoparasitic copepod Lernaeocera lusci (Bassett-Smith, 1896) was recovered from first-year bib (Trisopterus luscus L.) in the Voordelta (Southern Eight of the North Sea) from May until December 1989. Analysis of the seasonal abundance and of the population structure showed that transmission of infective stages to bib mainly occurred from June to September. From September to December the overall prevalence fluctuated around 70%. Maximum parasite population size (47/10(4)m(2)) and the highest total egg number were recorded in September and October, respectively. It was found that total parasite mortality was significantly influenced by mortality of hosts carrying parasites. Natural mortality probably contributed a small percentage to total parasite mortality. Calculation of the temporal mean-variance regression equation revealed that the parasites were aggregated within the definitive host population.