It has become customary to grow pathogenic organisms in the laboratory at 37°C. This practice seems to be based on the belief that those organisms "which have adapted themselves to a saprophytic or parasitic life in relation with warmblooded animals have an optimal temperature round 370C. to 38°C." (Andrewes, 1930). This belief does not appear to be based on any exact measurement of optimum growth temperatures for the various organisms concerned. In the case of Pasteurella pestis, at least, it has long been recognized by most workers that it grows "poorly above 35°C." though there is still a wide difference of opinion' as to the temperature at which optimum growth results. One of the present authors (Sokhey 1939b) had found the temperature for optimum growth in broth to be about 2700. More recently Spicer (1940) has reported that, "the optimum temperature for the growth of a Type III pneumococcus strain was found to be 27°C." In the case of P. pestis early workers, quoted by Albrecht and Gohn (1900), found the optimum growth to result at 37°C.; later workers, as reported in standard textbooks, put the optimum growth temperature at 25°C. to 300C., though Topley and Wilson (1936) state, "growth, however, at both 2400. and 37C. is often as good as at 30°C." Recently Tumanksy et al. (1935) made a comparative study of the growth of P. pestis and Pasteurella pseudotuberculosis Pfeiffer, at nine different temperatures beginning with 0°C. and ending with 4300. They found the optimum growth temperature of P. pestis to be 2800. to 300C. and the upper limit of growth at about 430C. Regarding the lower limit of the growth they found that the organisms showed some growth even at 00C. It would appear that at least some of the differences in the reported observations are due to different media, solid or liquid, employed, and the number of organisms used as the inoculum by different workers. In a previous paper (Sokhey 1939b), it was shown that on nutrient agar plates, if 5,000 organisms or more were seeded per square centimeter of the surface, more profuse growth resulted at 2700. than at 37.5°C., but if a smaller inoculum was used, i.e., 500 organisms per sq. cm. of the surface, no growth resulted at all at 2700. while some growth still appeared at 37.50C. after incubation for 48 hours. If blood agar was substituted for nutrient agar even with the smaller inoculum, 500 organisms per sq. cm. of surface, the number of colonies resulting both at 270C. and 37.50C., were equal in number, though the size of the colonies at 37.500. was smaller. In the present paper results are given of an investigation carried out to de-