This paper deals with the problem of diffraction radiation from an electric point charge that moves, at a constant speed, through a circular aperture in a plane screen. The screen is assumed to be electrically perfectly conducting. The problem is reduced to the solution of an integral equation for an unknown field function in the aperture. Low-frequency solutions as well as high-frequency solutions are obtained. Results pertaining to the farfield behaviour of the radiated field are given. Finally, numerical results pertaining to the radiation loss of the charge are presented.