The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F+ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent 27Al(I=5/2) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), gperpendicular to =2.0032(2), A/sub ///=1.665(2) mT, Aperpendicular to =1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent 23Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to pz transition with mean energy EF=4.15(5) eV. The corresponding s to px,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.