A comprehensive calcium carbonate map for the Indian Ocean surface sediments has been prepared from the analysis of about 1200 samples. The percent carbonate distribution shows the well-known first-order correlation with depth and hence reflects the physiographic peculiarities of the Indian Ocean. Thus on topographic highs such as most of the mid-Indian ridge, Ninety East ridge, etc. the carbonate is high (>75%), whereas in deep areas such as the Wharton, Central Indian, Madagascar, and Somali basins the carbonate is low (<10%). Dilutions by terrigenous debris are marked in the Bay of Bengal and Arabian Sea adjacent to the Indian subcontinent and in the Mozambique Basin off southeast Africa. When the regions with significant terrigenous dilutions are excluded, certain systematic latitudinal variations in the carbonate critical depth (CCrD, depth below which less than 10% CaCO3 is present) and carbonate lysocline (depth at which a significant decrease in carbonate percent occurs) superimposed on the general first-order depth changes are evident. The CCrD is deepest in the equatorial region (10¿N-10¿S). Southward it gradually becomes shallow and reaches 4800--4600 m in the 20¿S--30¿S region and 4800 m in the 30¿S--40¿S region. It tends to deepen (~4900 m) slightly in the 40¿S--50¿S region. The CCrD attains the shallowest level (3900 m) in the southernmost region (50¿S--60¿S) studied. The carbonate lysoclincal variation is in general opposite to that of the critical depths in the ocean north of 50¿S and is the same as the critical depth in the 50¿--60¿S region. These differences reflect the patterns of oceanic productivity and the dissolution effects of Antarctic Bottom Water. Low carbonate contents along with the presence of Antarctic diatoms and siliceous clay reported by Burckle et al. (1974) at 26¿S--29¿S and 60¿E--64¿E suggest a passage in the mid-Indian ridge in that area for the Antarctic Bottom Water flowing from the Crozet Basin to the Madagascar Basin.