The mesospheric ozone density at a height of 88 km has been deduced from simultaneous ground-based observations of neutral sodium density and sodium nightglow intensity, using a photochemical equilibrium model. The data have been obtained at S¿o Jos¿ dos Campos (23¿S, 46¿W). Sodium densities are measured as a function of height between 76 and 106 km by laser radar, and the nightglow intensities by a tilting filter photometer. The ozone density shows a strong seasonal variation with well defined maxima at the equinoxes, and a mean annual value of 8¿107 cm-3. The average nocturnal variation shows a factor of 2 increase between sunset and sunrise.