The chemical structure of mildiomycin (1) active against powdery mildews was determined by chemical degradations and physical analyses to be 2 - [(2R, 5S, 6S) - 2 - (4 - amino - 1, 2 - dihydro - 5 - hydroxymethyl - 2 - oxopyrimidin - 1 - yl) - 5,6 - dihydro - 5 - L - serylamino - 2H - pyran - 6 - yl] - 5 - (3H+ - guanidino) - 2,4 dihydroxyvalerate as shown in Chart 1. A new antifungal antibiotic mildiomycin (1) was isolated from the culture filtrate of Streptoverticillium rimofaciens B-98891. Mildiomycin shows strong activity against powdery mildews on various plants and remarkably low toxicity in mammals and fishes. This report deals with the structure of 1 in detail as shown in Chart 1.