An electron microscopic study of internal and surface-connected membrane systems of leg muscle of the crab shows that there are three kinds of surface-connected membrane systems in addition to an intracellular sarcoplasmic reticulum (SR). One is a system of large infoldings of the sarcolemma referred to as clefts. These are longitudinally-oriented, flattened infoldings of both the plasma membrane and the fibrous sheath of the fiber, and were probably seen earlier with the light microscope. Extending into the fiber both from these clefts and from the free fiber surface are two systems of tubules of much smaller caliber, the Z tubules and the A tubules. The Z tubules are located, as their name indicates, near the Z lines of the myofibrils, and are thought to be attached to them mechanically. The A tubules are found in pairs, near the ends of each A band, and are closely bound to the SR in two-part structures called dyads. Local-activation experiments, like those done earlier by Huxley and Taylor, suggest that the A tubules are involved in excitation-contraction coupling; no such experimental suggestion of function exists for the Z tubules.