There is a close correlation between the contraction speed of a muscle and its activity of actomyosin ATPase. In most mammalian muscles, two main types of muscle fibers can be identified, one with low actomyosin ATPase activity(type I) and the other with high actomyosin ATPase activity(type II). The former have a low constriction speed and the latter have a high one. There is also considerable variation in the laryngeal muscles. Accordingly, an enzymehistochemical study was carried out on intrinsic muscles of the canine larynx to see to what extent this functional difference is reflected in the proportions of type I and type II fibers. Muscle specimens of laryngeal intrinsic muscle were obtained from five dogs. Sections were stained with NADH-TR and ATPase at pH9.4, 4.65, and 4.3. Type I and IIA muscle fibers were identified, but type IIB fibers were absent. Each muscle had a type II fiber predominance of variable degree:cricothyroid had a mean ratio of type I to type II to be 47 : 53, posterior cricoarytenoid(49 : 51), whereas a pronounced type II fiber predominance was found in the thyroarytenoid muscles(33 : 67). The high percentage of fast-twitch, type II fibers in the thyroarytenoid muscles is well adapted to their emergency protective function, in which they rapidly constrict the glottis. By contrast, the higher percentage of type I fibers in the cricothyroid muscles may signify a greater performance as tensors of the vocal fold to determine high-pitched voices. Further, the abundance of type I fibers of the posterior cricoarytenoid muscle may be related to the fact that this muscle, as the only glottis abductor, is directly involved in respiration.