We computationally studied hyperpolarizability beta of C-2v fused-ring molecules substituted by two pairs of metaor para-positioned donor (D)-acceptor (A). In two types of meta-substituted isomers, the metal-type showed small beta(zzz), being plausible since the meta-positioned D-A pairs indicated the resonance structures not preferable for the large charge transfer (CT). Nevertheless, the meta2-type showed a large beta(zzz) resulting from a large forward CT associated with the HOMO-LUMO transition. This beta(zzz) increased with the number of fused-rings (the x-extension) and decreased slightly with small structural changes by the geometry optimization, but these changes did not vary the dominant CT structure. The x-extension was ineffective to the large beta(zzz) for the para-substituted isomer. (C) 2003 Elsevier B.V. All rights reserved.