SUMMARYIsolations were made of fungi from mushroom compost at the end of the colonization or growth phase (called spawn run in the mushroom industry) by mycelium ofAgaricus brunnescens.Fifty species of fungi, including many of the so-called weed-molds, were isolated over a period of 5 mo from 33 different lots of compost. Included were also fungi that previously have been reported as inducing high temperature within certain insulated substrates. These fungi were isolated from black areas of compost which had not been colonized by mycelium ofA. brunnescens, as indicated by a lack of white mycelial strands and by a failure of the compost to change in color from black to reddish brown. Determination of growth-temperature relations of these fungi showed that many could grow at a temperature above 33 C, a temperature which totally inhibits the growth of mycelium ofA. brunnescens.Thus some of these fungi may be inhibiting growth ofA. brunnescensin small localized areas of the compost because of their growth in that small area and the high temperature induced from their respiration.