An acceptor-related impurity was studied in bulk free standing GaN using electron paramagnetic resonance spectroscopy. Both undoped and Mg-doped substrates grown by the high nitrogen pressure solution method were examined, but only the doped sample revealed the presence of an EPR-detected center. Comparison with earlier work on p -type GaN films suggests that the defect is an acceptor, heavily compensated during growth. Photo-EPR studies revealed a photo-ionization threshold of 2.8 eV, which is interpreted as excitation of an electron from the acceptor to the conduction band. Based on the concentration and photo-threshold, the EPR center detected in the HNPS doped samples is tentatively identified as Be, which SIMS measurements indicate is a trace impurity in the doped GaN sample.