The International Indian Ocean Expedition (1960-65) has for the first time provided a sufficiently complete data set to support an investigation of the oceanographic and climatic seasonal influence on Indian Ocean thermal structure in the upper 500 m and north of 40 degrees S lat. Analysis of 23,847 bathythermograph and Nansen Cast measurements form the basis for a division of the Indian Ocean into 32 separate Primary Areas with distinct thermal features. The basic thermal parameters considered in the analysis and depicted graphically in this report include mixed-layer depth, maximum thermal gradient, five parameters of the permanent thermocline, and 2C-interval isotherm depths to 500 m. The thermal structure within each oceanographic region is qualitatively evaluated in terms of the relative effects of net surface heat exchange, heat advection, and circulation dynamics. (Author).