The aim of this study was to study the flexibility in prey selection of brown trout (Salmo trutta) by manipulating the abundance of drifting invertebrates in River Greånaälven in western Värmland, Sweden. This was accomplished by establishing three sites in the river where drift was artificially reduced using a drift net, three sites where drift was artificially increased by adding ants and three control sites where no drift manipulation was performed. The fish were captured by electrofishing, anaesthetised and stomachflushed. Analysis of their stomach content was performed at the laboratory.It was hypothesised that at increased availability of terrestrial drift, trout will include more terrestrial prey into their diets and that at reduced drift abundance, trout will exploit food resources on the substrate itself. Increased terrestrial drift had a significant effect on the diet. Ants became the most important prey in the brown trout diet during the manipulation. Approximately 50 %, by weight, of the diet consisted of ants. On the other hand, reduced drift abundance had no impact on diet composition. There was no significant difference between reduction and control sites. No correlation between drift composition and diet composition was found.