Sixteen photomultiplier tubes have been studied to determine the relation between the uniformity of their photocathodes and the resolution obtained from a &ggr;‐ray scintillation spectrometer using each of them. The sensitivity contours of the tubes have been plotted. The resolution was found to be related to theaveragesensitivity but could be improved by concentrating the light on the most sensitive part of the photocathode. Techniques for studying sensitivity with an artificial light flasher are described.