Chelae of decapod crustaceans are massive structures that often make up 35–50% of their total dry weight. In most cambarid crayfishes, males have longer and heavier chelae than females. Important functions of chelae of Orconectes propinquus include (1) capture and manipulation of prey, (2) defense against predators, (3) inter- and intra-specific interactions, and (4) reproductive activities. In the laboratory, males with large chelae were more likely to survive predation, occupy positions of dominance, and copulate with females, than males with small chelae. However, chelae appeared most important for reproductive activities (male–male interaction for females as well as sexual bouts with females) and secondary for prey manipulation and defense. In sexual encounters, males used their chelae to grasp and hold females during copulation. Because males with large chelae could interact more successfully with larger more fecund females, these males may contribute proportionally more genes to future populations than males with small chelae. Consequently, selection should favor large chelae in males. Because size of chelae is reduced in nonmating males during summer, I hypothesize that large chelae are only essential for reproductive activities and not necessarily required for effective prey manipulation or predator defense.