The nectar-feeding bat Glossophaga can be trained to discriminate two hollow forms, a hollow hemisphere and a paraboloid with the same diameter and depth, in total darkness. During training a saturation level of about 85-90% correct choices or more can be reached within 50-100 visits. To investigate generalization abilities, the bats were tested with pairs of the same shape but of different size. Although no reward was offered, they preferred the hollow sphere (30 mm and 50 mm diameter, but not 18 mm) over the corresponding paraboloids. Thus, the bats were able to generalize some features of the rewarded form and detect them in forms of the same shape but different size. This transposition is remarkable, since the bats could not use absolute spectral characters, but had to pay attention to size-independent features common to hollow hemispheres. Possible cues are the variation of echoes in dependence of different angles of calling direction (constant in spheres, changing with position in paraboloids) and/or the "timbre" of the echoes, i.e. their spectral pattern independent of their absolute pitch