A new screening method for inhibitors of cellulose biosynthesis is described. This method utilized three microbial strains; a cellulose-containing fungus Phytophthora, and a cellulose non-containing fungus Candida, and a bacterial strain of Acetobacter, a cellulose-producing acetic acid bacterium. The primary screen examined microbial cultures for selective growth inhibition against Phytophthora with no inhibition against Candida. The secondary screen tested for herbicidal activity. Thirdly, the active cultures were examined for their inhibition of cellulose biosynthesis by an Acetobacter strain. A screening trial with this new method led to the discovery of two microbial metabolites named phthoxazolin A and phthoramycin as new inhibitors of cellulose biosynthesis with herbicidal activity.