A model is presented for the formation of crystallographic tilting (misorientation) in highly mismatched (100) oriented zinc-blende semiconductor heteroepitaxial layers grown on vicinal substrates. The model is based on the asymmetric generation of 60 degrees misfit dislocations upon island coalescence in the initial growth process-and the asymmetric strain release at substrate steps, and predicts a correlation between tilt angle and initial growth planarity. Good agreement is found between model and experimental data on the GaAs/Si and other systems. It is also shown that, if the net tilt is small, an azimuthal rotation of the tilt axis may occur, even if the system symmetry would hinder it. The influence of thermal annealing and thermal mismatch on the tilt is discussed as well.