It has been suggested that an entomopathogenic bacterium, Xenorhabdus nematophila, exerts its pathogenicity against beet armyworm, Spodoptera exigua, by inhibiting phospholipase $A_2;(PLA_2)$ to shut down release of arachidonic acid, which is the primary substrate for eicosanoid biosynthetic pathway. This research aimed to prove the inhibitory hypothesis of X. nematophila on $PLA_2$ by directly quantifying arachidonic acid released from natural biological membrane. For determining optimal membrane preparation, fatty acid compositions from different tissues (hemocyte, fat body, and gut) were analyzed using gas chromatography in S. exigua. These tissue membranes showed different compositions in fatty acids, but possessed significant levels of arachidonic acid, amounting to ${approx};0.02\%$ among all fatty acids. Also, all cytosolic fractions of three tissues possessed significant and specific $PLA_2$ activity because these $PLA_2$ extracts catalyzed release of arachidonic acid from the membrane preparation into medium and were inhibited by a specific $PLA_2$ inhibitor, dexamethasone. For massive assay, we chose fat body as a tissue source of $PLA_2$ and substrate membrane because of its relative abundance. The effect of fat body $PLA_2$ extract on release of arachidonic acid increased with its concentration and incubation time. The fat body $PLA_2$ was susceptible to dexamethasone (a specific $PLA_2$ inhibitor) and p-bromophenacyl bromide (a secretory type $PLA_2$ inhibitor), but not to methylarachidonyl fluorophosphate (a cellular type $PLA_2$ inhibitor). X nematophila significantly inhibited fat body $PLA_2$ activity, but did not after heat-killing. An organic extract of X. nematophila culture broth also inhibited the fat body $PLA_2$ activity. These results indicate that S. exigua possesses arachidonic acid on the membrane and $PLA_2$ in tissues and that X. nematophila releases $PLA_2$ inhibitor(s) and inhibits $PLA_2$, which is susceptible to a secretory type $PLA_2$ inhibitor.