Electron-electron interactions give rise to the correction, Ï ∫(Ï), to the ac magnetoconductivity, Ï(Ï), of a clean 2D electron gas that is periodic in Ïc-1, where Ïc is the cyclotron frequency. Unlike conventional harmonics of the cyclotron resonance, which are periodic with Ï, this correction is periodic with Ï3/2. Oscillations in Ïint(Ï) develop at low magnetic fields, I cÏ, when the conventional harmonics are suppressed by the disorder. Their origin is a double backscattering of an electron from the impurity-induced Friedel oscillations. During the time Ï-1 between the two backscattering events the electron travels only a small portion of the Larmour circle.