ALLAN HILLS (ALH) 84001 is the most recently recognized(1) member of a suite of meteorites-the SNCs-that almost certainly originated on Mars(2). Several factors distinguish ALH84001 from the other SNC meteorites. Preliminary studies(3,4) suggest that it may be older than other martian meteorites. Moreover, it contains abundant, zoned domains of calcium-iron-magnesium carbonate that are indigenous to the sample(1) and thus may hold important clues regarding near-surface processes on Mars and the evolution of the martian atmosphere. We report here analyses of the carbon and oxygen stable-isotope Compositions of the carbonates that place constraints on their formation conditions. Our results imply the presence of at least two chemically distinct carbonates-one Ca,Fe-rich, the other Mg-rich-that are enriched in C-13 relative to terrestrial carbonates (delta(13)C approximate to + 41%parts per thousand), consistent with martian atmospheric CO2 as the carbon source. The oxygen isotope compositions of the carbonates indicate that they precipitated from a low-temperature fluid in the martian crust. Combined with textural and bulk geochemical considerations, the isotope data suggest that carbonate deposition took place in an open-system environment in which the ambient temperature fluctuated.