none A filter paper bioassay was used to determine sensitivity of laboratory strains of the warehouse pirate bug, Xylocoris flavipes (Reuter), and one of its prey, larvae of the red flour beetle, Tribolium castaneum (Herbst), to malathion. LD-50s for adult X. flavipes were estimated to be 51.4 mu-g (AI) per filter for males and 45.9 mu-g (AI) per filter for females. Larvae from a laboratory strain of T. castaneum with a mean weight of 0.42 +- 0.01 mg, similar to the weight of adult X. flavipes, had an LD-50 of 15.0 mu-g (AI) per filter. The predator X. flavipes was 4.3- and 10.2-fold more tolerant of malathion than laboratory strains of the parasitoids Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae) and Bracon hebetor (Say) (Hymenoptera: Braconidae), respectively. F-3-F-5 progeny of a field strain of X. flavipes collected from a bin of corn on a farm near Blackville, SC, in May and June 1993 were 33-fold (males) and 31-fold (females) resistant to malathion, relative to the laboratory strain, at the LD-50. Triphenyl phosphate, a carboxylesterase inhibitor, in combination with the LD-99 of malathion for the susceptible laboratory strain, abolished the resistance in both males and females. Larvae of T. castaneum with a mean weight of 0.37 +- 0.01 mg collected from the Blackville farm (F-2 progeny) were estimated to be apprxeq 740-fold resistant to malathion. We discuss development of the naturally occurring malathion resistance in X. flavipes, a generalist predator in the stored-product ecosystem, in term of the preadaptation and food limitation hypotheses.