A simplified one-dimensional quasilinear diffusion equation describing the time evolution of collisionless ions in the presence of ion cyclotron resonance heating (ICRH) and sources and losses is solved analytically for all harmonics of the ion cyclotron frequency. Simple time-dependent distribution functions which are initially Maxwellian and vanish at high energies are obtained and calculated numerically for the first four harmonics of resonance heating. It is found that the strongest ion-tail of the resulting anisotropic distribution function is driven by heating at the second harmonic followed by heating at the fundamental frequency. (author). 5 refs, 5 figs.