Laboratory and field experiments were used to investigate aspects of the behavioral ecological impact of the invading crayfish species, Orconectes rusticus. In aggression tests, individuals of O. rusticus dominated experienced O. virilis and O. propinquus but not naive individuals. In enclosures large O. rusticus killed smaller O. propinquus and brought about a shift in substrata selection in the laboratory. Size-matched sets of O. virilis and O. rusticus had similar effects on macrophytes in stream enclosures.