The structure of 1-(methylselanyl)-X-8(p-anisylselanyl)napthalene (2) and 1-(methylselanyl)-8-(p-chlorophen- ylselanyl)naphthalene (3) was studied by X-ray crystallographic analysis. The structures around the Se-Me and Se-C6H4OMe-p (Se-C-An) groups in 2 are close to type A and type B, respectively: type A if the Se-C bond is almost perpendicular to the naphthyl plane and type B when the Se-C bond is placed on the plane. Those around the Se-Me and Se-C6H4Cl-p (Se-C-Ar) groups in 3 are type B and type A, respectively. The nonbonded Se Se distances of 2 and 3 are 3.0951(8) and 3.1239(7) Angstrom, respectively. The structure of 3 is very different: from that observed in 1-(methylselanyl)-8-(phenylselanyl)naphthalene (1), the structure of which is type C, where the two Se-C bonds decline by about 45 degrees from the naphthyl plane. The structure of 3 strongly suggests the contribution of the nonbonded n(Se-Ar)...sigma*(Se-C-Me) three-center-four-electron (3c-4e) type interaction. The nonbonded n(Se-Me)...sigma*(Se-C-An) 3c-4e type interaction must partly contribute to the structure of 2. To clarify the nature of the n(Se)...sigma*(Se-C) 3c-4e type interaction observed in 2 and 3, together with the pi type two-center-four-electron interaction in 1, ab initio MO calculations with the B3LYP/ 6-311++G(3df,2pd) method were performed on model a, HaHbSe...SeHa'Hb', where the naphthylidene group was replaced by H-a and H-a' and the Me and Ar groups by Hb and H-b'. Two structures are optimized to be energy minima with