The levels of resistance to nystatin induced in 16 strains of Torulospsis glabrata by a few subcultures in the presence of the drug were high (minimum inhibitory concentrations > 20 000 U ml-1) compared with those obtained in 12 strains of Candida albicans (minimum inhibitory concentrations less than or equal to 2000 U ml-1). The results suggest a high rate of mutation in the first species.