Type I, I: B, II: A, and II: C fibers were identified by adenosine triphosphatase histochemistry in masseter muscles from 22 female pigs (1 year old, 70-90 kg body weight). Type II: B fibers were not found. This was in contrast to the findings of five fiber types in the porcine soleus muscles. In the porcine masseter the most prominent fiber type was II: A (75%). Type I fibers constituted 15% of the fiber types on average. Type I:B and II:C fibers were less frequent (4-6%). No significant difference was found between various biopsy locations, but there was a tendency towards more type I fibers in the deeper part of the masseter muscle. The mean fiber diameters were larger in the masseter muscles than in the soleus; however, the differences were significant only for fiber type I:B.□Histocytochemistry; masseter muscle; myosin adenosine triphosphatase