We conducted a series of experiments designed to investigate the regulation of regeneration following implantations of limb tissues in crabs. Autotransplantation (transplantation within the same crab) of the fourth walking leg tissues into claw stumps regenerated either a walking leglike limb, bifurcated limbs, a composite limb with claw and walking leg characteristics, or a normal claw. Interspecies experiments consisted of claw tissues fiom the crabCancer productustransplanted into the autotomized stump of the fourth walking leg ofCancer gracilis.The resulting regenerates appeared as either a claw-like limb, a walking leg with a toothed dactyl, a walking leg with two dactyls and two pollexes, or a normal walking leg. Reciprocal interspecies transplants of donor walking leg tissues into host claw stumps were able to induce the regeneration of walking leg-like limbs. Most of the regenerates had characteristics of both host and donor limbs. Our results suggest that both donor and host cells participate in limb regeneration. In the host claw stumps that produced the bifurcations, we propose that two distal organizers were generated. The identity of the limb fields generated by the distal organizers may be dependent upon the cells adjacent to the distal organizers, such that varying interactions between the two limb fields in the host stumps resulted in the formation of a range of regenerate morphologies.