Phacidin, an antibiotic produced by Potebniamyces balsamicola var. boycei, was tested for activity against some species of dermatophytes, yeasts pathogenic to man, opportunistic and systemic dimorphic fungi, using standard tube double dilution procedure. Results indicated that among dermatophytes, Trichophyton mentagrophytes, T. rubrum, and Epidermophyton floccosum were very susceptible (MFCs 1.56 μg/ml of the medium); the MFCs for the species of Microsporum ranged from 3.125 to 25 μg/ml. Scopulariopsis brevicaulis and Aspergillus nidulans gave the highest MFCs (400 μg/ml). For other species of opportunistic fungi, i.e. A fumigatus, A. niger, A. flavus, and A. terreus, the MFCs recorded were from 25 to 100 μg/ml. In the group of yeasts, which included six species of Candida, two species of Cryptococcus, one species of Saccharomyces, Torulopsis, and Trichosporon, the MFCs were in the range of 1.56 to 50 μg/ml. Surprisingly, the incitants of Blastomycosis, Coccidioidomycosis, Histoplasmosis, and Sporotrichosis were found to be very susceptible to phacidin (MFCs, 0.195 to 12.5 μg/ml).