Aggressive interactions, differences in chela size, and the effect of chela size on outcomes of aggressive interactions were studied in a laboratory setting using the federally protected (endangered) Orconectes shoupi (Nashville Crayfish), and two sympatric species, O. placidus (Bigclaw Crayfish) and O. durelli (Saddle Crayfish). Orconectes placidus and O. durelli are potential threats to O. shoupi through competitive or aggressive interactions. Understanding such interactions could help explain species distributions, provide insight on additional threats, and guide management decisions regarding Nashville Crayfish translocations. Aggressive interactions were examined with 30-min videotaped trials between body-size-matched hetero- and conspecific pairs. The predicted influence of chela size on outcomes of aggressive interactions was also analyzed. Our results demonstrated that O. shoupi males and females were significantly more aggressive than O. placidus. However, O. durelli females won more encounters and were slightly more aggressive than O. shoupi females. Significant differences in chela size were detected in some body-size-matched species and gender pairings: O. shoupi males had narrower chelae than O. durelli males; and O. shoupi females had longer and wider chelae than O. placidus females, and longer but narrower chelae than O. durelli females. Although chela size appeared to play a role in dominance, it was not the only factor influencing outcomes of aggressive interactions. Our laboratory results did not identify displacement threats to O. shoupi from O. placidus, and therefore do not preclude introduction of O. shoupi into habitat occupied by O. placidus to meet recovery plan objectives. However, interspecific aggression in the presence of a vital resource (e.g., food or shelter) was not tested here and should be investigated to provide a more comprehensive evaluation of possible threats to O. shoupi.