The biceps, semimembranosus, biceps femoris, and soleus muscles of female Rockland Wistar mice infected with either 1,000 Trichinella spiralis or 1,000 Trichinella pseudospiralis larvae were removed on days 12, 14, 16, and 18 post-infection (PI), sectioned and stained histochemically for their myosin ATPase activity. Light microscopic examination of the sections revealed that larvae of T. spiralis invade only the slow twitch muscle fibers, and those of T. pseudospiralis invade both the fast twitch and the slow twitch fibers. In sections obtained from mice infected with either parasite and killed on days 16 and 18 PI, identification of the majority of the infected fibers as fast twitch or slow twitch was not possible due to pathological modification of infected fibers.