Ground-based temperature and water vapor profiling of the atmosphere of Mars point to large annual variations in the water vapor saturation altitude, or hygropause, of the entire low- to middle-latitude atmosphere, as forced by the elliptical Mars orbit <Clancy et al., 1996>. We examine the affects of such an annual variation in the hygropause altitude (from 30 km) on the photochemistry of the Mars atmosphere. The one-dimensional, diffusive transport photochemical model of Nair et al. <1994> is run in a diurnally averaged mode for time-dependent calculations of the annual behavior of Mars photochemistry at low to middle latitudes. The model incorporates a specified annual variation of the water vapor profile, based on the microwave observations of Mars water vapor and temperature profiles versus season (solar longitude, Ls). Due to their long photochemical lifetimes, Mars CO and O2 are expected and found to be in significant non-equilibrium with the annually varying water vapor (and hence HOx) densities, and to present nearly constant abundances representative of the annual average water vapor profile. In contrast, Mars O3 has a photochemical lifetime of hours, and exhibits very large annual variations in response to the annual variation in the hygropause altitude. The global-scale abundance of O3 at altitudes of 20--40 km is predicted to vary from >109 cm-3 around Mars aphelion (northern spring/summer, Ls=71¿) to ~108 cm-3 around Mars perihelion (southern spring/summer, Ls=251¿). These model ozone variations are discussed in the context of the disparate Mars 5 (1974 <Krasnopolsky and Parshev, 1979>) and Phobos (1989 <Blamont and Chassefi¿re, 1993>) measurements of low-latitude ozone densities at 35--50 km altitude, as well as Hubble space telescope observations of enhanced low-latitude ozone during the 1995 aphelion of Mars <Clancy et al., this issue>. ¿ American Geophysical Union 1996