In the present paper, the regional distribution of calcium in the Atlantic water off South Brazil was examined oceanochemically by analyses of calcium contents in 78 water samples which had been collected off the coast of the São Paulo State. The chelatometric titration with EDTA reagent was employed in these analyses of calcium with good results. Calcium content of the water in the present area bears an almost direct relation to the chlorinity. The estimate of Ca/Cl is 21.56 X 10-3. Allowance has been made for the strontium included in the estimate for calcium. The estimate of Ca/Cl as mentioned above is much the same as that made by other researchers, and the Ca/Cl factor of the Atlantic water appears to be lower than that of the Pacific water. It is of great interest that the distribution of calcium in the water shows close correlation to the oceanographic condition in the present area of the South Atlantic Ocean. Namely, the surface water of the Brazil Current gave a relatively low estimate of Ca/Cl as 21.4 X 10-3 compared with that of the subsurface water, where it is 22.0 X 10-3 or more. Such a close relation between them certainly suggests that the Ca/Cl factor is an appreciable clue for distinguishing different water masses in a definite area. A characteristically minimum zone of calcium content was observed in the subsurface layer at the approximate depth of 50 m in the off-shore station (St. 69). This minimum zone can be attributed to the increased development of zooplankton populations, whose active respiration causes a high consumption of dissolved oxygen in a degree of saturation of 70% in the water of this zone. This distribution aspect of the calcium is noteworthy as it shows a close relation between calcium-utilizing organisms and their biochemical and ecological conditions.