The in vitro activity of polyenes was determined for 36 isolates of Zygomycota including Rhizopus spp. (15), Absidia corymbifera (10), Mucor spp. (6), Rhizomucor spp. (3), Cunninghamella bertholletiae (1) and Apophysomyces elegans (1). All isolates were tested with amphotericin B, heated amphotericin B and nystatin by a broth microdilution test. There were no significant differences between heated and unheated solutions of amphotericin B in terms of their antifungal activities. The MICs of amphotericin B for most of the strains were <2 mg/L. For all isolates, nystatin was significantly less active than amphotericin B (P < 0.001). The one C. bertholletiae and one A. elegans isolates were less susceptible to amphotericin B (MICs 2 mg/L) and were also less susceptible to nystatin.