1. The two chelipeds of the adult lobster are asymmetrical with respect to their external morphology, neuromuscular physiology and utilization in behavior; however, they are not genetically fixed in terms of placement or handedness. 2. The differentiation of muscle fiber types was studied in the cutter and crusher claw closer muscles in the early juvenile stages of the lobster Homarus americanus. Muscle fibers were characterized on the basis of sarcomere length. 3. In contrast to the adult lobster, where the claw closer muscles are asymmetric, the closer muscles of the stage 4 lobster are nearly symmetric; both short and long sarcomere muscle fibers are present in each claw and both fiber types have an identical regional distribution within the closer muscle. 4. By stage 6 one of the muscles differentiates into a cutter muscle with over 60% short sarcomere fibers and a distinct regional distribution of short and long sarcomere fibers. The other claw closer muscle slowly loses its short sarcomere muscle fibers and is transformed into a crusher claw, usually by stage 13-15. 5. The change of the closer muscles from a symmetric to an asymmetric condition is correlated with the loss in ability for the claws to undergo a "reversal" rather than with the external appearance of the claw which becomes differentiated several molts later.