Fifty-four single-spore isolates were obtained from two wheat field strains of Trichoderma koningii. These isolates inhibited the growth of Rhizoctonia solani to varying levels when tested on potato dextrose agar. 6-Pentyl-alpha-pyrone was isolated only from the extracts obtained from those isolates which showed strong inhibition of the pathogen on agar. Six isolates, three pyrone producers and three non-producers, were tested for protection of wheat against rhizoctonia root rot under controlled conditions. Only the pyrone producers significantly reduced root rot when applied to the soil and incubated for 14 d with the pathogen before planting. Those not producing pyrone did not reduce the disease when applied to the soil either with or without an incubation period in soil. There was no relationship between the disease protection ability of the isolates and their mycoparasitic ability or their production of chitinase, glucanase, cellulase or xylanase. This indicates that the pyrone antibiotic may have an import-ant role in the reduction of rhizoctonia root rot of wheat by the effective strains and that the ability to produce the antibiotic may vary among asexually produced progenies.