This paper summarizes work done in this laboratory over the last two years on the cloning of microsporidian rRNA by homology PCR and its subsequent use in diagnostic tests and phylogenetic studies. Using highly conserved primers in the 16S or small subunit rRNA (SSU-rRNA) these genes were cloned from human intestinal biopsies with transmission electron microscopy proven Enterocytozoon bieneusi and Septata intestinalis. The SSU-rRNA genes were then used to design and test several primer pairs for the diagnosis of microsporidian infection. Utilizing the polymerase chain reaction and primers V1 and EB45Ü Ent. bieneusi infected duodenal aspirates or intestinal biopsies could be detected. Using V I and SI500 infection with S. intestinalis could be detected. In addition to diagnostic tests, phylogenetic relationships were examined using sequence data from the fragment amplified by PCR by primer 530f in the SSU-rRNA and primer 580r in the large subunit rRNA. This data supported the placement of S. intestinalis in the family Encephalitozoonidae. In addition, it confirmed that Encephalitozoon cuniculi, E. hellem and S. intestinalis are distinct organisms. These techniques have broad applications to the study of other microsporidia and the development of a molecular phylogeny.