Myosin is composed of two high-molecular weight heavy chains and four low-molecular weight hght chains. In both vertebrate and invertebrate skeletal muscle, each myosin heavy chain is associated with two myosin light chains. In skeletal muscle myosins studied by X-ray diffraction, each myosin heavy chain binds one of each of two distinct classes of hght chains. Thus, while isoform distributions may vary within and between fibers, the spatial distribution of each class of light chain should be uniform within the A band and between sarcomeres and fibers. Since no such study exists for crustacean myosin, we investigated the spatial distribution of the hght chains within the superficial flexor muscle (SFM) of the lobster, Homarus americanus, using immunoelectron microscopy. The SFM contains two classes of myosin hght chains, termed “alpha” (Mr = 21,000 to 23,500) and “beta” (Mr = 18,000 to 18,500). Immunocytochemical electron microscopic results suggest that the alpha light chains are not uniformly distributed at the subsarcomere level.