We present a mathematical model that describes heat and mass transfer during solid-state fermentation (SSF) of Aspergillus oryzae in a well-mixed rotating drum bioreactor (RDB). In addition to the substrate bed and the headspace, the model recognises the bioreactor wall as a subsystem, allowing it to identify the role of this subsystem in heat removal from the bed. Model predictions agree well with previously published experimental data obtained in a rotating drum bioreactor of 0.19 m diameter and 0.85 m length, with maximum temperatures up to 15degreesC greater than the incubation temperature being reached. The model offers insights into the rate limiting steps in heat removal, and how SSF performance might be improved in the experimental system. (C) 2003 Society of Chemical Industry.