Abstract: The crude extracts from the antagonistic bacterium Pseudomonas aerugin
osa ZJ1999 were tested for effectiveness against Rhizoctonia solani, pathogen of rice sheath blight. Results showed that the inhibition ability against R. solani varied with concentration of the extracts and the treatment time for mycelium of R. solani. The colonization duration of the antagonistic bacterium on rice plants was closely related to the initial concentration. The population of P. aeruginosa ZJ1999 could retain at 10²-10³ cfu/g and 10³-10⁴ cfu/g 14 d after spraying on seedling and adult plants. Minimum concentration of the extract was 10⁸ cfu/ml, and the optimum spraying time was at the first day after inoculation of R. solan for best control of rice sheath blight.