We studied the in vitro and in vivo efficacies of the investigational drug isavuconazole against mucormycosis due to Rhizopus delemar. Isavuconazole was effective, with MIC and minimal fungicidal concentration (MFC) values ranging between 0.125 and 1.00 μg/ml. A high dose of isavuconazole prolonged the survival time and lowered the tissue fungal burden of cyclophosphamide/cortisone acetate-treated mice infected with R. delemar and was as effective as a high-dose liposomal amphotericin B treatment. These results support the further development of this azole against mucormycosis.