The vertical and radial betatron oscillation frequencies have been measured on the UCLA 50‐MeV spiral‐ridge cyclotron. The technique used was that of rf knockout, in which the internal beam was passed through a pair of exciter plates which provided an rf electric field. When the frequency was equal to the betatron frequency a detectable increase in betatron oscillation amplitude or ``knockout'' occurred. The knockout as a function of exciter frequency shows a number of resonances rather than a single one. This provides a measurement of radial as well as vertical betatron frequency. Comparisons of the measured frequencies with those calculated from magnetic field measurements are given and show satisfactory agreement.