The geometry and harmonic force field of the phenoxyl.radical are determined by ab initio calculations of the electronic structure. All the pi electrons are involved in strong nondynamical correlations, which are treated in a complete pi active space wave function. Results from using multiconfigurational self-consistent-field orbitals in the active space are found to be closely approximated by use of the more economical unrestricted Hartree-Fock natural orbitals. Large polarized basis sets are also required to obtain qualitatively correct results. Properties of the carbonyl chromophore are particularly interesting. Compared to closed-shell analogs, the equilibrium geometry indicates the CO bond in phenoxyl to be nearly as short as a typical double bond, whereas the scaled quantum-mechanical force field shows it to have strength intermediate between those of typical single and double bonds. The calculated fundamental vibrational frequencies generally confirm current interpretations of the major features of the observed experimental resonance Raman spectra, while leading to reassignments of some minor features. Some limited theoretical characterization of valence excited states provides qualitative explanation of the observed intensity pattern. Calculations carried out at lower levels of theory consistently find the CO bond to be too long and weak, leading to misassignment of the observed vibrational spectrum.