Studies on damp wheat, bin-burned under conditions simulating natural storage and under adiabatically controlled conditions, produced evidence which provides the basis for the belief that: (1) heating and bin-burning of damp grain during storage are caused primarily by microorganisms; (2) many species of bacteria and of fungi, normally present on grain, are associated with this spoilage; (3) these species are mesophilic, rather than thermophilic; (4) under natural storage, although the grain shows evidence of increased combustion, most of the heat is dissipated. Consequently, the temperature does not rise sufficiently high for thermophiles; and (5) if the temperature in isolated pockets in the mass rises high enough, the thermophiles probably are the result rather than the cause of the high temperature.