[en] A model is proposed for the origin of the tridiurnal variation observed by ground-based cosmic-ray detectors. The anisotropy is due to the first nonvanishing harmonic (beside the fundamental) in a sinusoidal expansion of the pitch-angle particle distribution in interplanetary space. The particle distribution is given by a perturbative-expansion of the pitch-angle scattering equation for quasi-state conditions in the solar wind. Observational implications of the model, including the amplitude and phase of the tridiurnal variation and its rigidity dependence, are discussed