We have studied magnetic properties and electronic structures of a novel persistent neutral radical, 2,5.8-tri-tert-butylphenalenyl(2), which is the first example of alternant hydrocarbon pi -radical isolated in the crystalline state. The observed magnetic susceptibilities of polycrystalline sample 2 were reproduced by assuming the thermal equilibrium between singlet ground state and excited triplet state of the dimer (2J/k(B)= -2x10(3) K) with traces of paramagnetic impurity molecules (0.3mol%). The pi -spin density distribution of radical 2 was determined by liquid-phase cw-ESR, H-1-ENDOR/TRIPLE, and paramagnetic H-1-NMR measurements. The origin of the large inter-molecular antiferromagnetic interaction in the crystal 2 was explained by an occurrence of effective SOMO-SOMO overlap between the two radicals on the six equivalent cc-positions with large positive spin densities of the carbon sites.