Measurements of the heat produced by Saccharomyces cerevisiae CBS 426 were used in conjunction with elemental and enthalpic balances to control fed-batch fermns. A proportional control using the ratio of carbon dioxide evolution rate to heat prodn. resulted in high biomass yields and minimal ethanol prodn. For the special case where the carbon source contains no nitrogen, biomass prodn. estd. from heat measurements agreed well with measured values. When the controller gain was set below the max. specific cellular growth rate, stable control was achieved, even in response to large upsets in feed concns. [on SciFinder (R)]