1. The P50 of Cancer Magister hemocyanin in hemolymph was 40.0 mm at 25 degrees C, nmax = 3.9; at 14 degrees C, 15.0 mm and 3.4; and at 10 degrees C. 10.2 mm and 3.3. 2. In the absence of Ca2+ and Mg2+, the P50 at 25 degrees C was in the range of 150 to 250 mm, depending upon pH. 3. Our results show that temperature variation and pH variation may be important factors in the physiological regulation of oxygen delivery in this species. 4. The automatic recording device of Imai et al. (1970) previously used for the study of hemoglobins, was found to be eminently suitable for the study of O2 affinities of hemocyanins.