The toxicity of 17 organophosphates to rats varied with the substituents surrounding the phosphorus atom and the route of administration. Compounds with the highest rat/house fly toxicity ratios were dimethoate, Dibrom®, (O, O-dimethyl-O-(1,2,- dibromo-2,2-dichloroethyl) phosphate), and Bayer 29493 (O, O- dimethyl (O) 4-methyl-mercapto-3-methylphcnyl phosphorothioate). Dimethoate, Bayer 29493, Bayer 23453 (O, O-dimethyl S-2-(ethylsulfinyl)ethyl phosphorodithioate), Bayer 24498 (O, O- dimethyl S-(methylsulfinyl)ethyl phosphorothioate), Bayer 25198 (O,O-dimethyl O-p-methylsulfinylphenyl phosphorothioate), and Shell SD 3562 (3-(dimethoxyphosphinyloxy)-N,N-dimethylcrotonamide) administered orally to rabbits were effective as animal systemics against the bed bug (Cimex lectularius L.) and Gulf Coast tick (Amblyomma maculatum Koch). The residual effectiveness (complete kill of test arthropods) of the systemically active esters was: dimethoate and Shell SD 3562, 2 hours, Bayer 24498, 4 hours; Bayer 25198, 6 hours; Bayer 23453 and Bayer 29493, 14 hours.