Trichoderma virens is an effective biocontrol agent for cotton seedling diseases incited by Rhizoctonia solani and Pythium ultimum. Strains of T. virens fall into two basic groups, the "Q" strains which produce the antibiotic gliotoxin and are more effective against R. solani and the "P" strains which produce the antibiotic gliovirin and are more effective against P. ultimum. "Q" strains can be selectively isolated from "P" strains and other soil fungi and bacteria by plating soil dilutions on a medium containing rifampicin (50 mg/ml), gliotoxin (40 mg/ml), and chlorothalonil (0.5 mg/ml) in PDA. "P" strains can be selectively isolated from "Q" strains and other soil microbes by plating dilutions on a medium containing rifampicin (50 mg/ml), chlorothalonil (1.25 mg/ml), and Thiabendazole (0.35 mg/ml) in PDA. T. virens strains already isolated from other sources may be separated into "P" and "Q" groups by the growth of "P" strains on PDA + Chlorothalonil (1.25 mg/ml) and "Q" strains on PDA + Maxim (20 mg/ml). Other Trichoderma species may be separated from T. virens by their growth on PDA containing Thiabendazole (0.4 mg/ml) or, with some species, 3% Yucca water extract (Cellucon Inc., Strathmore, CA) which inhibit the growth of T. virens.