A deep energy level of bromine in the bandgap of GaAs was determined by means of Radiotracer deep-level transient spectroscopy (DLTS) measurements. For this purpose, the radioactive isotope 77Br was implanted in p-type as well as in n-type GaAs. In the course of repeated DLTS measurements at n-type GaAs, the spectra are dominated by the occurrence of one bandgap state with the trap parameters Et=EC-0.45(2) eV and σ=2·10-14 cm2. This level vanishes on time scales of the nuclear half-life of the elemental transmutation from 77Br to 77Se (T1/2=57 h). Thereby, a definite correlation can be drawn between the observed bandgap state and a Br-correlated defect.