Aerobic incubation of soil from the Ap horizon of a Huron clay loam to which alfalfa as a carbon source was added resulted in rapid nitrification at 15 and 30 C and an apparent reduction in available carbon for denitrification. Use of the same soil in a subsequent study showed that aerobic incubation temperatures of 5, 10, 15 and 30 C did not influence its denitrification potential but that the level of available carbon probably did. Mean denitrification rates were 0.56, 1.23, 1.89 and at least 5.24 μg NO3−-N/g soil/h at 5, 10, 15 and 30 C, respectively, when carbon availability did not limit denitrification. At these rates, all NO3− applied or produced by nitrification, under normal soil fertility practices, could be denitrified within 1 or 2 days. Where readily available carbon was deficient, rates were 0.04, 0.18, 0.33 and 0.90 μg NO3−-N/g/h at 5, 10, 15 and 30 C, respectively. Reaction kinetics describing denitrification was of zero order when carbon was either available or seriously limiting; it was of first order where carbon availability changed from readily available to seriously limiting. Extrapolation of denitrification rates at various temperatures indicated that denitrification probably would occur at or near 0 C.