The effects of temperature on denitrification were studied in a soil system with a root in situ and one without a root. Soil samples of the Ap horizon of a Huron soil were used in the laboratory study. Germinated corn seedlings were planted and grown in cylinders for 84 h. At the end of this time the cylinders were cut horizontally so that the bottom 8.8-cm portion contained one root. Other cylinders were left unplanted. The soil in the cylinders was saturated with 5 ml of distilled water or 5 ml of a KNO3 solution. The rate of denitrification, both in terms of NO3− reduction and nitrogenous gas production, was more rapid in the system containing a root than in the fallowed system. Qualitatively, nitrogenous gases (N2, N2O and NO) were produced by both systems. At 30, 15 and 10 C there were no significant differences in the total quantity of gases produced by both systems. However, at all temperatures the crop system produced the larger quantities of NO, and also accumulated the larger quantities of NO2− and exchangeable NH4+. The root initially absorbed a portion of the added NO3−; however, with time the NO3− concentration of the root decreased to zero. The amount of NO3− taken up by the root and the rate at which the NO3− content of the root decreased was greatest at the high temperatures. At 5 C there was uptake of NO3− by the root but no decrease in the NO3− concentration of the root. Decreasing the temperature decreased the rate of denitrification, both in terms of NO3− reduction and nitrogenous gas production. At 5 C, denitrification was completely inhibited. Decreasing the temperature also changed the relative proportion of the nitrogenous gases produced. The quantity of N2 produced decreased as the temperature was decreased from 30 to 6–8 C. The decrease in temperature resulted in an increase in NO production; this was the principal gas produced at 6–8 C. Nitrous oxide (N2O) production, like NO, increased as the temperature decreased from 30 C to 10 C where it was the principal gas produced. At 30 to 15 C and 6–8 C, N2O production was secondary in quantity to N2 and NO, respectively.