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Fibrils from certain molluscan muscles, in particular the adductor muscles of the clam Venus mercenaria, were examined with the electron microscope and found to possess periodic variations in structure. In order to make these structural variations visible, it was necessary to treat the fibrils with reagents of high electron scattering power (electron stains). Phosphotungstic acid was found to be particularly suitable. This stain combines with specific regions in the fibrils, forming a remarkably regular geometrical pattern of which the most prominent feature is a regular cross striation, representing a fiber‐axis spacing of about 145A. Within each stained band, the stain is more highly concentrated in spots spaced about 193A from center to center across the band. A line drawn through any such spot parallel to the fiber axis passes through other similar spots, spaced five cross bands apart, making the length of the fiber‐axis period precisely five times the fiber‐axis spacing. X‐ray diffraction data obtained by Bear from the intact muscles are compared with the electron microscope observations. The small angle diffractions are in close agreement with those which would be expected from the observed structure except for the magnitude of the lateral spacing. Electron microscope values for this spacing are significantly smaller than the 325A indicated by the x‐ray data, probably as a result of lateral shrinkage in the vacuum‐dried electron microscope specimens. No significant difference in axis spacing has been observed in fibrils isolated from muscles fixed with alcohol in contracted and extended states.