The ultrastructure of normal and glycerol treated fibers of the closer muscle of the ghost crab, Ocypode cursor, was studiedmthe muscle is composed of presumably phasic (short sarcomeres) and tonic (long sarcomeres) fibers, the latter greatly predominating. Horseradish peroxidase (HRP) was used as an extracellular tracer to delineate the tubular system (TS), and to determine to what extent this system becomes detached from the extracellular space as a result of glycerol treatment. Sarcolemmal clefts invade deeply into the muscle at Z-lines and I-bands; tubules invaginate into the muscle from the clefts and from the surface sarcolemma at the Z-lines, A-I overlaps and A-bands. A tubules are in frequent diadic or tetradic contact with the sarcoplasmic reticulum (SR), whereas Z tubules appear to be randomly associated with SR, terminal cisterns (TC) and Z-line fibrils. When HRP was administered to normal muscle, black reaction product was found adjacent to the outer surface of the sarcolemma, within the clefts and within profiles of the TS throughout the tissue. In glycerol treated muscle peripheral vacuolation frequently occurred; black reaction product penetrated only as far as the vacuoles and into dilated Z-line tubules, but was virtually absent from the rest of the TS. This lack of continuity between the extracellular space and the A tubules indicated disruption or constriction of the A tubules as a result of glycerol treatment, although Z tubule contact with the extracellular space appeared unimpaired. These findings provide ultrastructural correlates of the electrophysiological changes produced by glycerol treatment of the closer muscle of the ghost crab (Papir, 1973), namely, interference with excitation-contraction (e-c) coupling. The random association of the Z tubules with SR and TC, and their resistance to disruption by glycerol treatment, tend to endorse the claims that the Z tubules in crustacean muscle are not directly involved in e-c coupling (Brandt et al., 1965; Peachey, 1967; Selverston, 1967).