SUMMARY: Temperature, salinity and pH optima of the marine bacterium Leucothrix mucor were determined directly in nature by use of tritiated thymidine auto-radiography and compared with the same characteristics of laboratory cultures. Field studies were done in Puget Sound, Washington, U.S.A., and Loch Ewe, Scotland. The temperature optima of cultures isolated from various sea-coast areas around the world were 28°, irrespective of the temperature of the habitat from which the culture was derived. In contrast, the temperature optima in the natural environment were significantly lower, ranging from 6·5° to 25°, depending on the habitat studied. Attempts to obtain physiological adaptation of laboratory cultures to low temperature failed. The results emphasize the danger of inferring the response to temperature of natural populations from the characteristics of laboratory cultures. In contrast, responses to salinity and pH optima in natural environments were the same as those of laboratory cultures.