The chelatometry - volumetric titration whith a chelate compound such as EDTA-2Na (disodium ethylenediaminetetracetate) - was applied to the determination of calcium in sea water. Sea water contains in general, approximately three times more magnesium than calcium. It is true that the chelatometry has presently given the possibility" of selective analysis of calcium in the coexistence of magnesium, but an abundant amount of coexisting magnesium, as in the case of sea water, is likely to cause an unfavorable negative-error of the titre in titrating calcium with EDTA using murexide indicator in a strongly alkaline medium. The author firstly examined how the negative error of the titre was affected by the probable conditions of the analysis; i.e., the quantity of coexisting magnenium, the hydrogen-ion concentration of the medium, and the addition of a masking reagent, as sucrose or sodium carbonate. These chemical examinations about the analytical conditions led to a procedure of analysis, by means of which it is possible to determine calcium in sea water easily, in a few minutes, with fairly high precision. As this method is very simple and gives an accuracy of ± 0.4% using only 10 ml of the water sample, it is recommended as a procedure to determine calcium in oceanographical observations aboard, in which a lot of water samples have to be analysed in a comparatively short period of time.