Abstract: A virulent strain BC-hn with high infectivity was isolated from naturally infected and dead Bactrocera cucurbitae collected from Hainan and evaluated for its insecticidal bioactivities.The strain BC-hn was identified as Serratia marcescens by morphological, cultural, physiological and biochemical tests, and 16S rDNA sequences analysis. Bioactivity of BC-hn(1×10⁸ cfu/mL)against B. cucurbitae and other insects were assayed in laboratory.Mortalities of B. cucurbitae adult, egg, larvae and pupa were 81.2%, 37.5%, 53.0% and 24.0% at 7 d post-treatment, respectively. When adults of B. cucurbitae were inoculated with concentrations of 1×10⁴—1×10⁸ cfu/mL of BC-hn, LC₅₀ was 1.16×10⁶ cfu/mL at 7 d post-treatment. The strain was also effective against Lipaphis erysimi larvae, resulting in a mortality of 60%, but not effective against larvae of Prodenia litura (Fabricius), Pieris rapae (Linnaeus), Pyrausta nubilalis (Hubern), Tenebrio molitor (L.), where mortalities were lower than 10%.These results show thatBC-hn is host specific in virulence and highly virulent against adults of B. cucurbitae.