This report describes a selective medium (MBV) that permits growth of Aphanomyces while inhibiting or restricting growth of several root-associated fungi that commonly obscure Aphanomyces on isolation plates. MBV contain the following ingredients per liter of water: 10 g of Difco-Bacto agar, 10 g of Difco cornmeal agar, 30 mg of metalaxyl, 5 mg of benomyl and 200 mg of Vancocin. MBV inhibits or greatly restricts growth of Fusarium, Rhizoctonia, Phytophthora and several Pythium species. Some isolates of Pythium were able to grow in the presence of 30 mg of metalaxyl but at rates lower than for Aphanomyces. If Alternaria, Mucor or Rhizopus interfere with isolation, 0.5 mg of amphotericin B can be used in the medium.