Introduced crayfishes represent a serious threat to the endangered aquatic fauna of North America and Europe. We used small-scale competition assay experiments to examine competitive hierarchies in the laboratory between male and female Orconectes rusticus and Orconectes illinoiensis, Orconectes indianensis, and Orconectes placidus from southern Illinois. These species are considered threatened in Illinois and there is a potential for O. rusticus to become established in drainages inhabited by these species. Using sex and size-matched laboratory competition trials for a limited food resource, we found that both male and female O. rusticus were competitively dominant to O. placidus, O. illinoiensis and O. indianensis (P < 0.05). Male O. placidus, O. illinoiensis, and O. indianensis displayed no competitive hierarchies (P > 0.05). Female O. placidus were, however, competitively dominant to O. illinoiensis which was dominant to O. indianensis (P < 0.05). These simple laboratory competition experiments have been used to explore competition hierarchies involved in both North American and European crayfish invasions to understand observed and potential invasion success. These experiments, although presenting only one small aspect of the many mechanisms involved in the invasion and displacement of a resident species, can aid in predicting the potential effects of introduced species prior to an introduction. Our work indicates that the invasion of O. rusticus could have negative consequences for native crayfish species in southern Illinois.