Four types of multiplier phototubes were selected to represent structures in common use and to allow comparisons of radiation sensitivity between tubes of different sizes and structures. Gamma fields ranging from a few rhr to more than 10 to the 10th power rhr with exposures of less than 10,000 r were of principal concern. The principal radiation sensitivity of all tubes was due to radiationinduced luminescence of the face plates and glass envelopes, and tube response increased with anode luminous sensitivity. The dynode structure and its configuration did not contribute significantly to the radiation-induced response. Radiation sensitivity can be reduced significantly through the use of non-luminescent materials in tube construction.