The relationship between structure and the insecticidal activity against American cockroaches, measured under synergistic conditions with defined amounts of metabolic inhibitors, was examined quantitatively for 80 meta-phenoxybenzyl ester-type pyrethroids. The compounds included various α-substituted iso-valerates and tert-butylacetates and various cyclopropanecarboxylates analogous to chrysanthemates and pyrethrates. Multiple regression analyses revealed that variations in the insecticidal potency are governed by variations in the following parameters: (i) the molecular hydrophobicity, log P, and the length of α-substituents represented by STERIMOL ΔL both of which exhibit a parabolic dependence; (ii) the receptor-binding enthalpy, Δ(ΔE), estimated by summing up the "average" binding energy values of functional groups involved in the molecule; (iii) ΔnDOF, the number of internal degrees of conformational freedom of which are lost on receptor binding of the molecule; (iv) the ionization potential of the acid moiety, IP, calculated by the AM1 semiempirical molecular orbital method; and (v) an indicator variable, IO, assigned for compounds with an ether-type oxygen atom at the α- or β-position of the α-substituent of the acid moiety. The optimum log P and ΔL values were about 7.0 and 6.9, respectively. The greater the Δ(ΔE) value and the lower the ΔnDOF and IP values, the higher was the insecticidal activity. The ether-type oxygen atom was not favorable to the activity, other things being equal.