Twenty-eight new oxa(thia)diazolyl 3(2H)-pyridazinone derivatives were synthesized. Some compounds showed good chronic growth activities against the armyworm, Pseudaletia separata (Walker). Their EC50 values were determined in vivo. Nineteen 2-tert-butyl-4-chloro-5-[5'-aryl-2'-(1',3',4'-oxadiazolyl)methoxy]-3(2H)-pyridazinones were quantitatively analyzed using the QSAR procedure. The partial least squares method was applied to find relationships between activity and the physicochemical data. The results showed that dipole moment, molar refractivity, and log P are identified as critical parameters for chronic growth effects.