The dilute bismuthide III-V semiconductor GaAs1−xBix alloys have unusual properties owing to large bowing of the band gap energy caused by Bi incorporation and a reduction of the temperature coefficients of the band gap. Deep-and shallow-level defects in device-quality GaAs1−xBix (x ≤ 5.4%) are investigated. Deep-and shallow-level defects behave as non-radiative recombination centers and electrical carrier traps. The Bi-related localized states induced by the interaction between spatially localized Bi states and the valence band of GaAs are continuously located up to ~90meV from the valence band with a density of ~1 × 10 17 cm −3. In spite of concerns about the degradation of the hole mobility in GaAs1−xBix due to scattering at these Bi-related localized states near the valence band, the p-type doping masks the contribution of the Bi-related states to the hole mobility, and a high hole mobility of 200 cm 2 V −1 s −1 is demonstrated. Despite low-temperature growth, the deep-level trap density in GaAs1−xBix is suppressed on the order of 10 15 cm −3 comparable to GaAs because of a surfactant-like effect of the Bi atoms. While the interface state density of ~8 × 10 11 cm −2 eV −1 in a GaAs/GaAs1−xBix heterointerface cannot be reduced by annealing, it can be reduced by half by the insertion of a Bi graded layer into the heterointerface.