The genetic homogeneity of nine commensal and infecting populations of Candida albicans has been assessed by fingerprinting multiple isolates from each population by Southern blot hybridization first with the Ca3 probe and then with the 0.98-kb C1 fragment of the Ca3 probe. The isolates from each population were highly related, demonstrating the clonal origin of each population, but each population contained minor variants, demonstrating microevolution. Variation in each case was limited to bands of the Ca3 fingerprint pattern which hybridized with the 0.98-kb C1 fragment. The C1 fragment was therefore sequenced and demonstrated to contain an RPS repetitive element. The C1 fragment also contained part or all of a true end of the RPS element. These results, therefore, demonstrate that most colonizing C. albicans populations in nonimmuno-suppressed patients are clonal, that microevolution can be detected in every colonizing population by C1 hybridization, and that C1 contains the repeat RPS element.