The following account records a number of observations and conclusions concerned with the structure of mammalian skeletal muscle, particularly as it has been visualized with the electron microscope. In part this work has depended upon the development of satisfactory techniques for sectioning material sufficiently thin for effective use with the electron microscope. Although conventionally fixed muscle has been studied to some extent, the principal work has been with material frozen in liquid air and lyophilized a t -72.OC. So far we have limited ourselves to magnifications not exceeding 10,000 diameters.The literature on the fine structure of striated muscle is tremendous. It is a subject about which there has been a great deal of speculation, as well as more o r less legitimate theoretical deduction. The latter, perforce, has often been based upon very limited information with inevitable possibilites that the assumptions were unjustified or incorrect. Our present findings indicate that a muscle fiber is constructed rather differently than has been supposed previously. Yet a detailed discussion in relation to previous work would be impractical. Therefore the reader is referred to Barer ('48) and to Fenn ('45) f o r recent reviews with extended bibliographies of the subject. Jordan ('33) aIso may be consulted. In addition, special attention must be called to the work of Hall, Jakus,