Cryptococcus neoformans isolates that exhibited unusual patterns of resistance to fluconazole and voriconazole were isolated from seven isolates from two different geographical regions: one isolate from an Israeli non-AIDS patient and six serial isolates from an Italian AIDS patient who had suffered six recurrent episodes of cryptococcal meningitis. Each isolate produced cultures with heterogeneous compositions in which most of the cells were susceptible, but cells highly resistant to fluconazole (MICs, 64 µg/ml) were recovered at a variable frequency (7 × 10³ to 4.6 × 10²). Evidence showed that this type of resistance is innate and is unrelated to drug exposure since the Israeli patient had never been treated with azoles or any other antimycotic agents. Analysis of clonal subpopulations of these two strains showed that they exhibited heterogeneous patterns of resistance. The number of subpopulations which grew on fluconazole or voriconazole agar declined progressively with increasing azole concentration without a sharp cutoff point. For the Italian serial isolates, the number of clonal populations resistant to fluconazole (64 µg/ml) and voriconazole (1 µg/ml) increased steadily, yielding the highest number for the isolate from the last episode. Attempts to purify a sensitive subpopulation failed, but clones highly resistant to fluconazole (100 µg/ml) and moderately resistant to voriconazole (1 µg/ml) always produced a homogeneous population of resistant cells. Upon maintenance on drug-free medium, however, the majority of the homogeneously resistant cells of these subclones lost their resistance and returned to the stable initial heteroresistant phenotype. The pattern of heteroresistance was not affected by the pH or osmolarity of the medium but was influenced by temperature. The resistance appeared to be suppressed at 35°C and was completely abolished at 40°C. Although heterogeneity in azole resistance among subpopulations of single isolates has been reported for Candida species, the transient changes in expression of resistance under different growth conditions reported here have not been observed in fungal pathogens.