The role of heavy ions in the collisionless parallel shock of Venus and Mars is investigated where the solar wind interacts directly with the planetary ionosphere. We consider O+ ions in the planetary plasma and He++ ions (&agr; particles) in the solar wind as the heavy-ion species. By solving a linear dispersion relation of the ion cyclotron beam instability, which provides a dissipation in the parallel shock, we find that the dominant frequency and wavenumber are not much modified when the heavy ions are included in a realistic amount. Particle simulations are performed in order to examine the effects and dynamical behavior of the heavy ions in the parallel shock. The re-formation of the parallel shock is observed, which confirms that the electromagnetic waves that make up the parallel shock is not affected by the heavy ions. Notable pickup of O+ ions by electromagnetic waves is not detected. Instead, strong heating of the solar wind He++ ions is observed in the parallel shock region. ¿ American Geophysical Union 1996