The structures and energies of neutral benzenoid and neutral quinonoid oligo(p-phenyls) and of oligo(p-phenyl) dications and dianions were calculated at the ab initio SCF level for oligomers containing up to 11 rings. Compared to analogous theoretical data as obtained earlier on oligathiophenes, we compute a spatial extension of the positive and negative bipolaronic defects in poly(p-phenylene) of only about three rings, thus considerably shorter than in polythiophene. The shorter defect extension in poly(p-phenylene) correlates also with the finding that even if one attempts to force the quinonoid structure by the addition of appropriate terminal groups, a transition to the aromatic benzenoid structure occurs already in the center of chains as short as about six rings.