Within a field experiment in a small stream we investigated the impact of benthivorous fish on the feeding behaviour of Gammarus pulex with consequences on its fitness. We compared a fish containing stretch with a fishless reference stretch. In the presence of benthivorous fish we expected two changes in the foraging behaviour of G. pulex because of its hiding in leafs and a low locomotion activity: 1) A reduction of the general food intake, and 2) a switch from animal prey rich in proteins and lipids to poor-quality food, detritus and leafs. Higher mobility in the fishless reference is expected to increase the chance to feed on animal prey with the consequence of higher fitness and lipid store. The gut fullness of G. pulex measured over 24 hours was similar in the fish and fishless stretches. However, triglyceride concentrations of females and the egg quality were higher in the fishless reference. Final results of stable isotope analysis (N, C) will show if differences in food composition lead to these consequences.