Abstract: The inhibitory effects of cell-free supernatant (CFS）produced by four strains of Rhodospirillaceae (Rps.capsulata PSB-2, Rps.sphaeroides PSB-3, Rps.gelatinosa H1 and Rps.palustris H10） against pathogenic bacterium were assayed. And the influences of aquaculture environmental factors on antagonistic effects of metabolic products were studied. The results show that antagonistic materials of four strains of Rhodospillaceae were secreted after the stationary phase, and cell-free supernatant (CFS） had inhibitory ability against more than twenty pathogenic bacteria. As for inhibitory activity， Rps.capsulata had the strongest antibacterial activity among four strains of Rhodospirillaceae, and its MIC against pathogenic bacteria was 8～64AU/mL. MIC of other three strains of Rhodospirillaceae were 4～16AU/mL. Antibacterial activities of Rhodospirillaceae were affected significantly by pH value and the concentrations of NH4+-N and NO2?-N: antimicrobial effects were greatly significant between pH 7 and pH 9, and under the concentrations of NH4+-N below 5 mg/L and NO2?-N below 7 mg/L, but antimicrobial activities were not sensitive to the changes of temperature and Dissolved Oxygen (DO) in aquaculture water.