A systematic investigation of deep levels in undoped and doped (Fe, Fe/V, excess of Bi) single crystals of Bi12 GeO20 (BGO) has been performed by photoinduced current transient spectroscopy. A list of traps in the energy range 0.1–1.0 eV with their thermal characteristics and their concentrations has been drawn up. Ten different levels have been detected, a few of them subdividing probably into substructures. In all probability, these trap species are present simultaneously in all the crystals but their concentrations differ considerably (typically from less than 1012 up to 5×1015 cm-3 ) so that dominant levels emerge. Variations in the number, nature, and concentration of the dominant centers were observed from preparation to preparation even for undoped BGO. This makes it difficult to draw reliable conclusions concerning the effect of doping, though some of the observed changes are probably directly related to Fe or Fe/V doping. This study of BGO also enabled us to test the experimental tool that we have worked out and progressively refined.