A sterol mutant was isolated after nitrosoguanidine treatment, on the basis of th nystatin-resistant phenotypic behaviour, from a Candida albicans strain requiring adenine. The plasma membrane ultrastructure of the resistant mutant, observed by freeze-fracture electron microscopy, was not significantly different from the parental, ergosterol producing nystatin-sensitive strain. Nystatin treatment caused no ultrastructural change in the resistant strain, but led to the aggregation of membrane particles in the sensitive strain: this effect was considered to be the specific effect of nystatin. Other, probably nonspecific effects of nystatin on the sensitive strain were a deepening and deformation of invaginations, atypical membrane fracture, certain changes in the structure of cell wall and special ornamentation of its surface. No change in the intracellular membranes was observed.