Ab initio crystal orbital calculations have been performed on stereoregular polymethineimine (PMI) at the restricted Hartree-Fock/6-31G level. Since the optimized geometries of the parent oligomers show only small energy differences, four isomers, namely, trans-transoid (TT), cis-transoid (CT), trans-cisoid (TC) and helical (HEL) have been studied to point out the geometry-dependences of the band structure, density of states (DOS) and polarizability. Some of these could be helpful for identifying the isomers. It turns out that the DOS makes the TT and CT isomers distinguishable, whereas the TC and HEL isomers present very similar DOS. Moreover, it is shown that the longitudinal polarizability per unit cell. increases in the order HEL, TC, CT and TT. [S0021-9606(98)52303-3].