Continuous culture was achieved in several cell lines of a microsporidium obtained from the skeletal muscle of an AIDS patient. Development in COS-1 and RK13 cells was prolific. Spores from the original biopsy were also inoculated into athymic mice by i.m. and i.p. routes. Infection was found in several organs as well as in skeletal muscle after a few weeks. All stages were surrounded by an electron-dense surface coat. Meronts had 2-4 nuclei and divided by binary fission. In sporogony the surface coat became separated from the plasma membrane to form a sporophorous vesicle, within which division into sporoblasts was effected by repeated binary fissions. The number of sporoblasts (and later spores) within the sporophorous vesicles varied from 2 to > 32 and the sizes of the vesicles varied, according to the number of spores contained therein, from 5 microns diameter to 14.0 x 11.0 microns. Spores measured 4.0 x 2.4 microns and had a prominent posterior vacuole. The parasite differs from the genus Pleistophora in that it does not form multinucleate sporogonial plasmodia and that the sporophorous vesicle enlarges during sporogony and its wall is not a multilayered structure. It is proposed to place it in a new genus and species Trachipleistophora hominis n.g., n.sp.