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It is shown that the transit time in a photoconductor cannot be less than the charge relaxation or storage time if the photoconductor has contacts of the ``space charge,'' or ohmic, type. The principal consequence of the observation is that the photoconductor cannot simultaneously act as a detector, an amplifier, and a storage element and still have a response time as short as the storage time. This puts a restriction on the performance of high‐resistivity photoconductive devices. For example, with present television standards and acceptable response times, the maximum quantum efficiency possible for a photoconductive camera tube of the vidicon type with space charge limited dark current is less than unity. [Note added in proof.—These same general conclusions have also been reached by A. Rose, Helv. Phys. Acta 30, 242–244 (1957)].