Rostrum dace (Leuciscus leuciscus burdigalensis) from the River Viaur were found to be infested with the ectoparasite Tracheliastes polycolpus (Copepoda: Lernaeopodidae). Samples from five study sites along the river revealed different patterns of parasite infestation. Heavily infested fish were found at the upper study sites whereas much lower infestation levels were observed at the lower study sites. The copepods showed an aggregated dispersion pattern on host fins. The results showed significantly preferred microhabitats, with adult females being more abundant on the anal, pelvic and along the external part of the pectoral fins. The anal and pelvic fins were damaged by the parasite with a loss of their surface area. These fin alterations may reduce the fish's swimming ability and therefore affect the rostrum dace population. Our findings highlight the need to study the effects of parasites on stream fish populations.