The production of antibiotic substances by Serratia plymuthica A21-4 was greatly enhanced by modifying components of a growth medium. When the minimal medium containing K2HPO4 0.7%, KH2PO4 0.2%,(NH4)2SO4 0.1%, MgSO4 0.01% was used as basal medium, the best carbon source for antibiotic production was glycerol and the most favorable nitrogen source was ammonium sulfate. The modified medium for antibiotic production also increased colonization ability of A21-4 on pepper root and in the rhizosphere soil. When the cells of A21-4 were suspended in modified medium, the population density of A21-4 on pepper root was 10-100 times higher than that suspended in 0.1 M MgSO4. The population density of A21-4 on root did not decrease under 106 cfu/groot up to 21 days after treatment although the inoculum of A21-4 was reduced to 107 cell/ml. Similar tendency was also observed in the rhizosphere soil. Consequently, Phytophthora blight of pepper was successfully controlled by A21-4 with 107 cell/ml suspended in the modified buffer solution instead of 109 cfu/ml suspended in 0.1 M MgSO4.