Abstract: The antagonism of An A₃ strain of Bacillus pumillus to the hyphae and conidia of the paulownia canker pathoses, Valsa paulowniae was tested with plate culture method. The results showed that the strain had strong antagonistic effects to the hyphae and conidia, with 3-4 cm in diameter of the inhabiting circle on the PDA plate inoculated with the A3 strain and the pathoses. Field experiments were carried out in the past three years on the control effect of fermented broth of the A₃ strain and the fermented broth mixed with carbendazim against the canker. When the canker spots were scraped off and the tree trunk was treated with the broth or the mixture, 97.8% and 99.4% of control were achieved, respectively. Microscopic observations showed that treatment with the A₃ strain of B. pumillus caused lysis of hyphae and conidia resulting from hypertrophying and malforming, cell wall breakdown and leaking of the cytoplasm.