Geometry optimizations of the lowest (second) excited singlet states of the fulvalene systems were carried out by using the ab initio MCSCF method with the 6-31G(d) basis set. The electronically excited molecules are found to undergo a pseudo-Jahn-Teller (JT) distortion from D-2h to C-2 nu On the basis of the results obtained, an energy component analysis of the total energy has been carried out to elucidate the physical picture of the pseudo-JT effect. Inspection of the energy components comprised in the total energy reveals that the stability of a less symmetric structure (C-2 nu) is attributable commonly to the energy lowerings of the intenuclear repulsion term and the interelectronic repulsive and kinetic terms due to sigma electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT bond distortion. It is further found that in triafulvalene and heptafulvalene the nuclear-electron attractive and interelectronic repulsive terms due to pi electrons contribute to the pseudo-JT stabilization, respectively. In pentafulvalene and 7,7＇-bis(bicyclo[4. 1.0]hepta-1,3,5-trienylidene), on the other hand, both the nuclear-electron attractive and interelectronic repulsive terms due to pi electrons contribute to the energetic stability of the C-2 nu structure. Subsequently, heptafulvalene undergoes a pseudo-JT effect from the planar C-2 nu to a nonplanar C-s structure. Interestingly, the stability of nonplanar C-s structure results from the energy lowering of the nuclear-electron attractive term. Moreover, the structural and electronic properties inherent in the less symmetric C-2 nu and C-s nuclear arrangements of the electronically excited molecules are discussed.