A technique is described for the radioautography of whole mounts of muscle fibers, which are approximately 15µ in diameter and too small to be handled individually. The fixed muscle is digested with collagenase to separate the fibers; these are then stained with hematoxylin, thoroughly washed and suspended in radioautographic emulsion on microscope slides. Thus the nuclei, which are all at the surface of the fibers, are close to the surrounding emulsion. This ensures that all of the nuclei containing the radioisotope will be labeled in the radioautograph, irrespective of their position, above, below or at the side of the fibers.