A model of the ion chemistry and dynamics of the ionosphere of Mars at large solar zenith angles has been constructed using techniques previously developed for Venus. It is found that a substantial nighttime ionosphere should occur even when the ionopause height is as low as 250 km. At low altitudes the interaction of the plasma flow with the neutral atmosphere appears to result in considerable ''noise'' in the flow patterns. Transterminator plasma velocities are smaller than for Venus, although they may exceed the local sonic speed. The effect of a nocturnal source of suprathermal electrons is also discussed. ¿ Copyright 1990 by the American Geophysical Union