Abstract: A virulent strain PS-1 with high infectivity was isolated from the natural death larvae of Phyllotreta striolata. The strain PS-1 was characterized as Serratia marcescens based on its morphological, cultural, biolog identification system and 16S rDNA sequences analysis. The larvae of P. striolata were fed with turnip cubes which had been dipped in suspension culture of different concentrations (3×10⁴, 3×10⁶and 3×10⁸ cfu/mL) of S. marcescens PS-1 for 24 h. The corrected mortality rates were 36.1%, 63.9% and 75.0% at 4 d post-treatment, respectively. The LC₅₀was 7.98×10⁶, 4.27×10⁶and 8.02×10⁵cfu/mL at 2, 3 and 4 d. The corrected mortality rate of P. striolata larvae fed with cabbage leaves which had been dipped in suspension culture of S. marcescens PS-1 under the concentrations of 1×10⁸ cfu/mL was 23.25% at 12 d post-treatment. When the eggs of P. striolata were dipped in suspension culture of different concentrations (1×10⁵, 1×10⁷ and 1×10⁹ cfu/mL) of S. marcescens PS-1, there was no significant difference in the hatching rate between the control and treatments. The results suggest that S. marcescens PS-1 which has a strong virulence against the larvae and adults of P. striolata might be a good candidate as a potential microbial agent for biological control.