The influences of Cd on nitrification rate and the population of nitrifying autotrophs in sandy clay loam soil were studied. The soil was treated with 0, 5, 50 or 500 μg of Cd per g dry soil and preincubated. After preincubation for 32 days, 100μg of NH^+_4 -N per g dry soil was applied to the preincubated soil, and the oxidation of ammonium and changes in number of nitrifying autotrophs were assayed. The determination of NH^+_4 -N and (NO^-_2 + NO^-_2) - N revealed that, though the rate of nitrification was not decreased below the control level by the application of 5 and 50μg Cd/g dry soil, the nitrification was markedly retarded by 500μg Cd/g dry soil. However, the number of ammonium-oxidizing autotrophs in the soil treated with 500μg Cd/g dry soil was larger than those found in the other soils. The number of nitrite-oxidizing autotrophs was smallest in the soil treated with 500μg Cd/g dry soil. These facts suggest that nitrification rate is controlled, in certain environmental conditions, not only by the number of the nitrifying autotrophs but also by the nitrifying activity of the individual microorganisms.