Characteristics of the Hamamatsu R1635 10‐mm‐diam photomultiplier have been measured. Some typical photomultiplier characteristics—such as gain, dark current, transit, and rise times—are compared with data provided by the manufacturer. Photomultiplier characteristics, generally not available from the manufacturer, such as the single photoelectron time spread and pulse response for full photocathode illumination were measured and are discussed.