<p>From the histochemical study of the intercostal muscles of cat, the following results were obtained. 1. Three different types of muscle fibers have been clearly distinguished in intercostal muscles by histochemical demonstration of succinic dehydrogenase; namely, the white fibers are stained faintly, while the red fibers deep blue and the intermediate fibers purple or bluish purple. 2. The difference in these stains is due to the degree of the enzyme activity, i. e., the areas of high SDH activity are stained deep blue while those of relatively low SDH activity are stained purple. 3. At oil immersion magnification, the differences among the three types of fibers are clearly distinguishable by the amount, size, distribution pattern and subsarcolemmal precipitation of Nitro-BT formazan particles. 4. Concerning the spatial distribution of these three types of fibers in each intercostal muscle, the muscles in the cranial and caudal parts of thorax (I-IV, VIII-XII) show a higher proportion of red fibers, while those in the middle thorax show a higher proportion of white fibers. 5. The vertebral portion of the first internal intercostal muscle is composed of only two types of fibers, red and intermediate ones, and their diameters are almost the same in size as in soleus muscle. In the middle intercostal muscle (V-VII), an intimate relationship can clearly be observed between the size and the enzyme activity of muscle fibers as in the gastrocnemius muscle. 6. In comparison with the anatomy of thorax and the distribution of muscle fibers, it may be presumed that there is a close relationship between the distribution and the scope of thorax movements, however, no definite relation between the distribution pattern and respiratory participation of muscle fibers. 7. Hence, it appears that the intercostal muscles in the cranial and caudal parts of thorax perform original respiratory movements, while the muscles in the middle thorax mainly perform voluntary respiratory movements, perhaps display their function during forced breathing. The intermediate fibers may usually have some tonus and carry out the role of resisting ribs from falling inside by negative pressure of the thoracic cavity.</p>