The thermally excited triplet state of p-phenylenebis(phenylmethylene) (p-PBPM) which is a topological isomer of the first organic high-spin molecule, m-phenylenebis(phenylmethylene) (m-PBPM, S=2), was examined using single crystal H-1-ENDOR (Electron-Nuclear DOuble Resonance) spectroscopy. From the H-1-ENDOR spectrum of the triplet state we found that the absolute sign of the fine-structure parameter D was negative and that p-PBPM in the triplet state has eight negative pi-carbon sites and six positive ones. The pi-spin density distribution determined experimentally indicates that the carbon atoms of the central p-phenylene ring have negative pi-spin density. The spin-spin interaction between the induced pi-spins causes the fine-structure parameter D to be negative. It is concluded that the pi-pi spin interaction predominates in the triplet state of p-PBPM.