This paper describes a simple inexpensive laboratory stream which is used to detect possible side-effects of pesticides on stream invertebrates. Gammarus fasciatus, an important crustacean in river food-chains, tends to migrate up-current which makes it subject to possible drift effects. Data are presented on the effects of 24 hr. LD10 levels of temephos (Abate) and chlorpyrifos (Dursban) on the capacity of this organism to migrate upstream.