none PESTAB. The effects of Orthene (cacephate) insecticide were evaluated against a variety of organisms. It was found to combine low mammalian toxicity with high systemic efficacy (with transport in the xylem) and safety to the braconid parasitoid, Chelonus blackburni. In addition, it was found to be highly stable to hydrolysis and ultraviolet degradation. Subsequent metabolism and penetration studies indicated that the observed toxicity of Orthene could usually be explained on the basis of Monitor (methamidophos) production and degradation. Penetration was a factor in Orthene resistance only in the pupal stages of C. blackburni and its host the potato tuberworm and, to a lesser extent, in the O-(2,4-dichlorophenyl) O-ethyl phosphoroamidothioate resistant house fly. O- and S-demethylation of Orthene before it could be metabolically converted to Monitor also contributed to resistance in many organisms. Excised cotton leaves produced detectable amounts of Monitor, as well as O-demethylated Orthene from Orthene taken up in the water solution through the cut petiole.