We present the first three-dimensional model simulations of ozone on Mars. The model couples a state-of-the-art gas-phase photochemical package to the general circulation model developed at Laboratoire de M¿t¿orologie Dynamique (LMD). The results do not contradict the classical picture of a global anticorrelation between the ozone (O3) and water vapor columns. However, the quantitative approach shows significant departures from this relationship, related to substantial orbital variations in the O3 vertical distribution. Over the period Ls = 180¿--330¿, low-latitude to midlatitude O3 is essentially confined below 20 km, has a weak diurnal cycle, and is largely modulated by topography. During the rest of the year (Ls = 330¿--180¿) the model predicts the formation of an O3 layer at 25--70 km altitude, characterized by nighttime densities about one order of magnitude larger than during the day. Throughout the year, high-latitude O3 peaks near the surface and reaches maximum integrated amounts (~40 ¿m-atm) in the winter polar vortex, with considerable (30 to 50%) dynamically induced day-to-day variations. The most stringent comparison to date with O3 observational data reveals contrasted results. A good quantitative agreement is found in the postperihelion period (Ls = 290¿--10¿), but the model fails to reproduce O3 columns as large as those measured near aphelion (Ls = 61¿--67¿). Current uncertainties in absorption cross sections and gas-phase kinetics data do not seem to provide credible explanations to explain this discrepancy, which may suggest the existence of heterogeneous processes.