Prostaglandin levels were determined by fluorometric HPLC analysis of hemolymph collected from larvae of the true armyworm, Pseudaletia unipuncta, that had been injected with bacteria. Prostaglandins were extracted and derivatized with the fluorogenic compound 9-anthryldiazomethane and detected by fluorescence-HPLC. One of the prostaglandins produced was identified as prostaglandin F 2a based on HPLC retention time. The chemical identity of prostaglandin F 2a was confirmed by isolation and derivatization followed by gas chromatography mass spectrometry analysis. Larvae injected with heat-killed bacteria, Serratia marcescens, produced about 4 times as much prostaglandin F 2a as larvae injected with saline. In a separate experiment, larvae injected with bacteria and the prostaglandin precursor arachidonic acid produced still higher levels of prostaglandin F 2a . The production of prostaglandin was inhibited with phenidone, a dual cyclooxygenase and lipoxygenase inhibitor. These data indicate that bacterial injections stimulate increased eicosanoid biosynthesis in true armyworms, particularly biosynthesis of prostaglandin F 2a . Our findings add considerable support to the hypothesis that eicosanoids mediate insect cellular immune reactions to bacterial infections. Arch. Insect Biochem.