In vitro susceptibilities of 78 isolates of pathogenic filamentous fungi to the imidazole compounds R34,000 and miconazole and to amphotericin B were determined using an agar dilution technique. Allescheria boydii, Sporothrix schenckii and the dematiaceous fungi (Cladosporium, Fonsecaea and Phialophora spp.) were most susceptible to miconazole with minimum inhibitory concentration (MIC) values ranging from less than or equal to 0.25 to 32 microgram/ml and with geometric mean MIC (G-MIC) values of 0.56-1.24 microgram/ml. Isolates of Aspergillus fumigatus were more susceptible to amphotericin B and miconazole than to R34,000 with G-MIC values of 1.20 and 3.48 microgram/ml, respectively. Coccidioides immitis, Histoplasma capsulatum and Blastomyces dermatitidis were highly susceptible to all three drugs (G-MICs greater than 1 microgram/ml); R34,000 was the most active of the three compounds against C. immitis (G-MIC = 0.44 microgram/ml).