Abstract: An Antarctic bacterium P4-11 was isolated from Antarctic seawater.Its molecular identification was determined using 16 S rRNA sequence analysis,its biocontrol effect on greenhouse cucumber powdery mildew was determined and both acute dermal test and oral toxicities on the guppy were also studied.The results show that the strain P4-11 belongs to Pseudomonas species.Biocontrol effect on cucumber powdery mildew was 68.26\% with the fermentation broth dilution 1:1, and the control effect was 66.57\% with 1:2 dilution,higher than that of 63.71\% with 1\% Wuyi streptozotocin 500-fold dilution.Toxicity tests showed the level of acute dermal toxicity and acute oral toxicity of strain P4-11 fermentation broth was low,no abnormalities were detected on guppy.The strain has potential applications in biological control of plant diseases.