SUMMARYThermophilic and thermotolerant fungi were quantitatively isolated from soil samples collected near Bloomington, Indiana, on eight separate dates throughout the summer of 1975. Samples were collected from each of four different field sites which differed in amount of sun heating and in amendment with organic matter. Eighteen species of thermophilic and thermotolerant fungi were isolated. Fungal growth is described on the basis of observation of in situ growth and results of plate counts. Growth rates of thermophilic and mesophilic soil fungi were measured in vitro in soil, alone and in pairs, at 20 and 37 C, and at fluctuating temperatures. The role of sun-heating of temperate soil in allowing growth of thermophilic and thermotolerant fungi, and in inhibiting growth of mesophilic fungi, is evaluated in view of the in vitro and in situ data presented. Evidence is presented in support of the conclusions that the high minimum temperature for growth of thermophilic fungi is not a laboratory artifact, and that tolerance to microbial antagonism is not necessarily low in thermophilic fungi. No evidence was found that production of antagonistic chemical compounds by thermophilic fungi was enhanced by exposure of the species tested to temperatures lower than those required for growth.