Numerous strains of pseudomonads formed growth inhibition zones around their colonies which had been placed on lawns of phytopathogenic bacteria and fungi used as indicators. The antimicrobial activity spectra against the indicator bacteria and fungi varied greatly with the species and strain of the pseudomonads, suggesting that various antimicrobial substances participated in the antagonism. Strain ATCC7700 of Pseudomonas aeruginosa was highly antagonistic in vitro to various plant pathogenic bacteria and fungi, especially to Ralstonia solanacearum. Treating of tomato roots with P. aeruginosa at the time of transplanting in R. solanacearum-infested soil increased the percentage of seedling survival. Soaking the root systems of tomato seedlings in a bacterial suspension of ca. 10¹⁰ cfu/ml resulted in the highest suppressiveness. Pretreating tomato roots with killed cells of P. aeruginosa also gave protection, suggesting that mechanisms other than antibiotic production, such as induced resistance and infection sites competition were involved in suppression of the disease.