Microsporidia are obligate intracellular protozoa that parasitize both vertebrates and invertebrates and are now recognized as important pathogens in individuals infected with human immunodeficiency virus type 1 (HIV-1). We describe the clinical and morphological features of a case of pleistophora (microsporidian) myositis in a patient with AIDS and delineate the stages of the microsporidian life cycle, as visualized by light and electron microscopy. We discuss significant aspects of microsporidian infections in humans and of myopathy attributable to other causes in HIV-1-infected individuals. As far as we know, ours is only the second reported case of microsporidian myositis and the first reported case in a patient with documented HIV-1 infection.