Abstract: The beet armyworm, Spodoptera exigua, is one of the serious vegetable pests in China. In this study, toxicity of virulent Serratia marcescens isolate PS-1 originating from the larvae of Phyllotreta striolata were determined against S. exigua under laboratory conditions. The second instars of S. exigua were fed for 48 h with an artificial diet which had been dipped in a suspension culture of PS-1. The treated larvae exhibited mortality in a dose dependent manner. Larval mortality significantly increased with increase in PS-1 concentration in the range of 2×10¹-2×10⁸cfu/g, and pupal survival and adult fecundity showed obvious decrease. Population growth of S. exigua was decreased by 74.4%-100% compared to the control. When S. exigua adults were fed for 2 d with honey solutions containing PS-1 at 1×10³, 1×10⁵and 1×10⁷cfu/mL, longevity of treated adults decreased by 5.70, 5.91 and 7.16 d, respectively, and fecundity was significantly reduced. When the artificial diet was dipped in supernatants of PS-1 suspension cultures (1×10⁹cfu/mL) and used for the second instars for 48 h, larval mortality reached 70% in 8 days, which was higher than that in the control. Life table analysis demonstrated that PS-1 had a strong toxic effect on larvae and adults of S. exigua. These results show that S. marcescens PS-1 is a potential biocontrol agent against S. exigua.