After ultraviolet light (UV) irradiation of 2-nitrobiphenyl (2-NBP) below 120 K, stable spin-triplet species can be observed in rigid glasses and in the crystalline state by the electron paramagnetic resonance (EPR) method. The zero-field splitting (ZFS) parameters obtained from the UV-irradiated single crystal of 2-NBP at 77 K are X/hc = 0.0144, Y/hc = 0.0222, and Z/hc = -0.0367 cm(-1). The absolute values of these ZFS parameters are relatively larger than the corresponding ones obtained in ethanol glasses. To determine the absolute signs of the ZFS parameters and the energy of singlet-triplet separation, the temperature dependence of the EPR signals was observed between 1.93 and 4.2 K. Taking Boltzmann populations and the EPR transition probabilities into account, we found that the triplet state observed is 1.7 +/- 0.5 cm(-1) above the singlet ground state. The triplet species is inferred to be formed through the abstraction of a hydrogen atom from the 2’-position of the biphenyl ring by one of the oxygen atoms of the nitro group by taking the direction of the principal spin axes and the result of a semiempirical molecular orbital calculation into consideration.