In gammaridean Amphipoda from the river Slack in northern France larval stages of two Acanthocephala species have been found. In Gammarus fossarum larvae of Polymorphus minutus, a parasite of waterfowl, occurred. This worm was found only once in Echinogammarus berilloni. Larvae of Metechinorhynchus truttae, parasitizing fish, were harboured only by E. berilloni and have not been encountered in Gammarus fossarum during the present investigations.