Quasi-linear theory predicts that ions in resonance with transverse ion cyclotron waves suffer merely pitch angle diffusion while conserving their total kinetic energy in the frame moving with the wave phase speed. For the first time, direct observational evidence from Helios plasma data is shown for the occurrence of this pitch angle diffusion of solar wind protons, induced by resonance with parallel ion cyclotron waves propagating away from the Sun. Parts of the isodensity contours in velocity space are well outlined by a sequence of segments of circles centered at the adapted wave phase speed, which is assumed to vary slightly and to be due to dispersion smaller than the local Alfv¿n speed. This observation confirms the validity of basic concepts of resonant wave-particle interactions as described by quasi-linear theory. The solar wind proton velocity distributions show a plateau defined by a vanishing pitch angle gradient in the resonant regime, implying marginal stability of the distribution function. The implications of these results for solar wind ion heating and kinetic transport are discussed. ¿ 2001 American Geophysical Union