Th determine whether Candida albieans in patients with AIDS represents a unique strain, C. albicans isolated from 24 patients with AIDS was compared with Candida isolated from 23 healthy adults. Resistance to 5-fluorocytosine, synthesis of amino acids and nucleotides, sugar use, and enzyme activity patterns were similar among isolates from the two groups. Molecular analysis revealed similar banding patterns of EcoRI restriction fragments of DNA between 2.5–3 and 6–7 kb. In addition, the frequency of a dimorphic 3.7-versus 4.2-kb band, identified by agarose gel electrophoresis and by probing Southern transfers of EeoRI digests with a cloned fragment of C. albieans DNA encoding 25S ribosomal RNA, was not significantly different between the AIDS-derived and control C. albicans. C. albicans isolated at different time points in the course of disease and from different sites in individual patients showed identical DNA fingerprints. The similarity in isolates of C. albicans that cause disease in AIDS patients and those present in healthy subjects suggests that the candidiasis associated with AIDS is not due to the presence ofa unique or particularly virulent strain but is likely the consequence of a defect in host defense mechanisms. © 1990, by The University of Chicago.