An upper temperature limit near 60 degrees for eukaryotic organisms is documented by results of a systematic search for fungi able to grow at higher temperatures. Samples from hot springs, thermal soils, self-heating coal waste piles, and other natural and man-made heated habitats did not yield fungi when enrichments were done at 62 degrees , whereas fungi able to grow at 55-60 degrees can be readily isolated from such habitats. Earlier work had shown that eukaryotic algae are also absent from environments with temperatures above 55-60 degrees . It is suggested that the failure of eukaryotes to evolve members able to grow at higher temperatures is due to their inability to form organellar membranes that are both thermostable and functional.