In his electronmicroscopic observations conducted on the red muscle fibers and white muscle fiders with the use of M. soleus and M. gastrocnemius in rabbits and cats, the author obtained the following results. 1. The source of functional differentiation in the red muscle fibers and white muscle fibers exists only in sarcoplasm, and no marked differences can be seen between the red and white muscle fibers as regards their myofibrils, nuclei, and sarcolemmas. 2. Mitochondria of the red muscle fibers are either round or elliptical in shape and are dense and numerous in the muscle fibers; while mitochondria of the white muscle fibers are slender and extremely less in number. 3. Mitochondrial cristae of the red muscle fibers are projecting like a cog-wheel but few in number, while those of the white muscle fibers are arranged at right angle to the long axis of the mitochondria and forming a dense crease. 4. Judging from the relationship between the shape and number of mitochondria as well as from the state of cristae, there seems to exist an intermediary type of muscle fibers between the red and white muscle fibers. 5. The sarcoplasmic reticnlum is well developed in the white muscle fibers. 6. The structural differences of sarcoplasm in these two muscle fibers coincide well with the differences in the function and energy metabolism of the two.