The insecticidal, miticidal, and ovicidal activities of 5 formamidines, including chlordimeform, were examined in laboratory studies against eggs and adults of the twospotted spider mite, Tetranychus urticae Koch, carmine spider mite, T. cinnabarinus (Boisduval), and citrus red mite, Panonychus citri (McGregor), mobile forms of the green peach aphid, Myzus persicae (Sulzer), and eggs and early-stage larvae of the southern armyworm, Spodoptera eridania (Cramer), cabbage looper, Trichoplusia ni (Hubner), bollworm, Heliothis zea (Boddie), and tobacco budworm, H. virescens (F.). In general, excellent ovicidal and larvicidal activity was associated with N′-(2-methyl-4-chlorophenyl) substituted formamidines while the N′-(2,4-dimethylphenyl) analogs were significantly more toxic to adult miles and aphids. In response to treatment by the formamidines, adult mites displayed distinctive behavioral patterns: one, “walkoff” was associated with the nature of the formamidine, the other, “spindown” with the arylthio substituent present on 2 of the compounds. Chemical modifications of insecticidally active parent N-aryl-N-methylformamidines have resulted in the production of more toxic compounds.