Fiddler crabs present an accentuated sexual dimorphism wherein males have one of the chelipeds greatly enlarged, compared to the other chela or to the two small feeding chelipeds of females. Available observational data in the literature suggest a dual function for this trait both as an armament to fight other males and as an ornament to court females by claw waving. We experimentally tested femaleUca tangerifor possible preferences based on the size and elevation (waving posture) of the male large cheliped and on male handedness. Dead, resin-coated males were used as test objects. Females significantly more often approached males with larger claws and males with raised claws. Male handedness had no effect on female responses. These results are consistent with a female mating preference based on claw size and position and hence with the evolution of claw size and the waving display as a result of sexual selection by female choice.