Between the spring of 1999 and the spring of 2002, 196 conger eels, Conger conger, were collected from the Ría de Arousa, Galicia, Spain, and examined for hemiurid flukes. Five trematode species belonging to the family Hemiuridae were found: Hemiurus communis, Lecithochirium fusiforme, Lecithochirium rufoviride, Lecithochirium furcolabiatum, and Lecithochirium musculus. The 2 most common species were L. fusiforme and L. rufoviride, each with a prevalence of more than 80%. Furthermore, both species occurred simultaneously in 145 conger eels (74%). In contrast to the population of L. rufoviride, the mean intensity and abundance of L. fusiforme revealed a clear seasonal pattern, with peak values coinciding with each of the 3 winters studied. However, the prevalence of L. fusiforme did not increase significantly during winter. These differences may be related to an aggregated distribution of the parasite. The results show that infrapopulations of this species fluctuate greatly in the host with time and suggest that this parasite matures relatively quickly but does not survive for much longer than 3 mo in conger eels.