Pseudomonas fluorescens strain CHA0 is an effective biocontrol agent of diseases caused by soilborne plant pathogens. Strain CHA0 produces several toxic metabolites, notably cyanide, acetylphloroglucinols and pyoluteorin. By means of genetically engineered derivatives of P. fluorescens, cyanide has been shown to be an important factor in the suppression of black root rot of tobacco caused by Thielaviopsis basicola. In addition to cyanide, 2,4-diacetylphloroglucinol has been suggested to be involved in the suppression of this disease. Strain CHA625, which was obtained after Tn5 mutagenesis, did not produce 2,4-diacetylphloroglucinol and suppressed Gaeumannomyces graminis var. tritici-induced take-all of wheat to a distinctly smaller extent than did wild-type CHA0 under gnotobiotic conditions. A cosmid, pME3101, obtained from a genomic library of strain CHA0, restored the ability of strain CHA625 to produce this metabolite and partially restored its suppressive capacity. 2,4-Diacetylphloroglucinol was shown to be produced by strains CHA0 and CHA625/pME3101 but not by strain CHA625 in the rhizosphere of wheat, grown under gnotobiotic conditions. This compound was more toxic to the growth of G. graminis var. tritici than to the growth of wheat in vitro. These results suggest that the production of 2,4-diacetylphloroglucinol by P. fluorescens strain CHA0 plays an important role in the suppression of take-all of wheat.