Successful biocontrol of Pythium induced cotton seedling disease with Gliocladium virens is dependent on the strain of the b control agent and the kind of substrate on which it is grown. When strain GV-P of G. virens, grown on a millet substrate, was air dried, ground to 500 micron particle size, and coated on to cotton seed with latex sticker, it controlled seedling disease caused by Pythium ultitnum. Strain GV-P grown on other substrates, and other strains of G. virens grown on millet, did not control the disease. Strainsubstrate interactions were assayed for biocontrol activity with a soil test tube method. Moist cotton field soil, infested with P. ultimuin oospores and incubated for six weeks, was added in 5 gram lots to test tubes, and a single cotton seed was planted in each tube. The planted tubes were incubated in the dark for 7 days at 15 C, then transferred to a lighted bench and held at 22 C for 5 days. The emerged and surviving seedlings were then examined and counted. Disease levels obtained by this method were highly consistent from test to test, making