SUMMARYA simple mechanistic model is developed for predicting the fate of radioactive caesium in the soil beneath grassland. A continuity equation is derived describing the diffusion, leaching and reaction of caesium in soil, with uptake by a uniformly distributed root system. The soil reaction is characterized by three phases: an instantaneous exchange, a moderately‐fast reversible reaction and a slow reversible reaction. The time‐dependent reactions and root uptake have first‐order kinetics. It is shown that caesium is unlikely to be removed from the root zone by leaching and diffusion, even under prolonged high rainfall. Therefore, since the rates of the soil reactions and root uptake are much greater than the rates of diffusion and leaching, it is not necessary to calculate the concentration profile to find the rate of uptake. The computer program for the model is just 30 lines offortran, and requires only a few seconds of micro‐computer CPU time for each year of simulation.Some predicted rates of plant uptake and distributions in the soil are given. It is seen that the fate of the caesium is markedly influenced by soil parameters dependent on soil clay content, clay mineralogy, organic matter content, cation saturation and pH, and by the rooting density. The predicted rates of uptake vary from about 0.5% to 50% per annum, which is compatible with data reported elsewhere. The predictions are intended to expose gaps in knowledge of the fate of caesium in soil: in particular, the mobility of sorbed radio‐nuclides on soil surfaces needs further study. Predictions are not given for the fate of caesium in any real situations because insufficient data are