A Red soil (pH 5.0) from Bangladesh without and with addition of 100 p.p.m. N as ammonium sulphate was mixed with 0.1 per cent sulphur and calcium carbonate (0.3 and 0.6 per cent levels) and incubated at 30°C and 50 per cent saturation (aerobic) for 13 weeks. Mineral-N accumulation due to incubation increased with level of added calcium carbonate, but was hardly affected by the sulphur treatment. Mineralized N accumulated entirely as nitrate in the controls and with both levels of calcium carbonate, but entirely as ammonium where sulphur was added. There was no accumulation of nitrite. At low pH nitrification may be due to acid-adapted strains of autotrophic nitrifiers and/or to heterotrophic nitrifiers.