The age and fabric of 347 Phanerozoic marine oolites have been compiled in order to evaluate secular variations in the original mineralogy of these physicochemically precipitated sedimentary carbonates. Variation in the abundance of oolite correlates with positions of global sea level; ooids were common during transgressions and regressions but were rare during lowstands and highstands. The rarity of oolite during continental emergence (early Cambrian, Permo-Triassic, late Tertiary-Quaternary) probably relates to a diminished areal extent of shallow-carbonate environments. Causes for a similar paucity during continental submergence (Silurian-Devonian, late Cretaceous) are more problematic; higher atmospheric CO 2 and lower hydrospheric CO 3 (super =) concentrations during highstands may have inhibited a biotic carbonate precipitation. Ooids coeval with highstands display a preponderance of fabrics evincing a primary calcite mineralogy; ooids formed during lowstands are more diverse but record dominance of aragonite. Variations in cortical mineralogy relate more closely to flooded continental areas than to sea-level elevations, supporting the suggestion of MacKenzie and Pigott ( 1981 ) that variations in hydrospheric carbonate concentrations were more important in determining Phanerozoic ooid mineralogy than were magnesium-calcium concentrations. These data on oolite abundance and mineralogy demonstrate that abiotic Phanerozoic marine carbonates have been responsive to secular changes in atmospheric-hydrospheric chemistry and global eustasy, and suggest that these changes are directly related to past rates of submarine weathering and sea-floor hydrothermal activity.