An electron paramagnetic resonance (EPR) signal, labeled TU1, has been found in hydrogenated Czochralski-grown Si crystals after irradiation with fast electrons and annealing at 300-400 °C. An isotropic g factor of 1.9987 indicates the shallow donor nature of the defect giving rise to the signal. Unusual for shallow donor centers in silicon, well-resolved lines due to the hyperfine interaction of unpaired electrons with 29Si nuclei are detected in the TU1 spectrum. The observed features are expected if a wave function of the unpaired electron is localized around a central Si atom of the defect. From a comparison of the EPR data with the results of optical and electrical measurements on identical crystals, it has been established that the TU1 signal is related to the shallow donor state of a hydrogen-oxygen-related center with negative-U properties.