SummaryIn a red latosol loam under bare fallow at Kawanda, Uganda, nitrate accumulates to as much as 70 p.p.m, nitrate nitrogen during short periods of dry weather. Nitrate accumulation appears to be a microbiological process involving nitrifying bacteria similar toNitrosomonasandNitrobacter.Nitrate accumulation in an isolated topsoil, 3 in. deep, ceased when the moisture content fell below 10 per cent. Accumulation continued on plots which were in contact with the subsoil and which remained at 10 per cent. moisture.It was concluded that under the conditions of these experiments, transport of ions to the surface by capillary transport or diffusion does occur, but is not responsible for the major nitrate accumulation, which seems to depend on progressive drying of the topsoil during which nitrate, as formed, becomes protected by the dry conditions from microbial reduction, assimilation, and leaching. Any process which prevents the progressive drying, e.g. shading, mulching, and watering, can be expected to favour nitrate removal and so to prevent nitrate accumulation.