A database of pre-Quaternary Phanerozoic reefs is used to test the significance of ancient reefs as paleoclimatic tracers. The compilation of reef paleolatitudes through time and comparison with published paleoclimate curves shows that neither the width of the tropical reef zone nor the total latitudinal range of reefs is correlated with published estimates of paleotemperature. However, reefs trace paleoclimate indirectly: Algal reefs tend to prevail during icehouse climatic intervals, and distinct high-latitude reefs are only developed in cold intervals, whereas during greenhouse episodes, the reef zone usually ended abruptly at a particular latitudinal boundary in the subtropics. Additionally, different biotic reef types tended to be concentrated in different latitudes. The lowest latitudes have usually been occupied by coralline sponge or microbial reefs, coral reefs tended to grow in intermediate latitudes, and bryozoan reefs constantly occupied the highest latitudinal position.