The distribution of molecular tritium (HT) oxidation activity and HT oxidizing bacteria in 5-cm soil sections from the surface to 20 cm depth in natural and cultivated fields in Mito was determined in in vitro experiments. HT oxidation activity was the highest in the top section of the natural soil, about twice that of the top section of the cultivated soil, and decreased with depth. From the natural and cultivated soil sections, 195 and 969 isolated strains with HT oxidation activity were obtained, respectively. The distribution profile of the occurrence rate and the sum of oxidation activity of HT oxidizing bacteria in each soil section were consistent with that of HT oxidation activity in the soil section. Most of the HT oxidizing isolates, 84% for the natural soil and 94% for the cultivated soil, were actinomycetes, Gram-positive bacteria.