Changes in the dark current and noise pulse spectrum of photomultiplier tubes due to ionizing radiations are reported. Tubes studied were the RCA 7265 and various versions of the ``ruggedized'' RCA 6199. Measurements were made both at 25°C and −40°C under exposure to60Co &ggr; rays, cosmic rays,241Am alpha particles, and90Sr &bgr; rays. All ionizing radiations incident on the phototube face were observed to produce additional dark current, and a considerable enhancement of the pulse spectrum, particularly at pulse sizes corresponding to several cathode photoelectrons. Each cosmic ray and interacting60Co &ggr; ray produced one large pulse, equivalent to about 10 single photoelectrons, and about 10 smaller pulses. The241Am and90Sr produced less than one noise event per incident particle. In order to explain the effects, three mechanisms seem to be required. Cˇerenkov radiation will produce the large pulses due to relativistic cosmic rays and Compton scattered electrons by60Co &ggr; rays. Scintillations in the glass with an efficiency of about 10 photons/MeV produce the effects due to the nonrelativistic alpha and &bgr; particles. It is suggested that phosphorescence induced by the short wavelength Cˇerenkov quanta can account for the many small pulses observed. These effects significantly alter the signal‐to‐noise characteristics of phototubes on balloons and satellites when applied to the detection of weak light sources, or low intensity x rays in the environment of space.