The relative numbers of microorganisms associated with compost during mushroom production were studied by the dilution plate method. Thermophilic actinomycetes and fungi were isolated A4th a very high frequency early in the growing season. Although numbers of thermophilic bacteria diminished slowly during the season, the thermophilic fungi and actinomycetes diminished rapidly with the latter disappearing after 6 weeks. Mesophilic fungi other than Agaricus or Trichoderma remained relatively

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... e throughout the growing period. Agaricus could be isolated between the first and third break. Trichoderma became dominant after the fourth break. The mesophilic bacterial counts diminished during the most productive portion of the mushroom cropping season and then increased to much higher numbers toward the end of the season.