Monocultures of spring wheat (Triticum aestivum L.) were grown at overcrowded densities (10 000 and 3 000plants per m2) under well-watered and water-stressed conditions to investigate the effects of water deficits on self-thinning. The results showed that density reduction in water-stressed populations was delayed compared with that in well-watered populations. Populations grown in well-watered conditions conformed to the -3/2-power law. Compared with the well-watered condition, there was no significant decrease of the self-thinning line under water-stressed conditions in this experiment, although the rate of average shoot biomass accumulation decreased, This result implied that the exponent of the -3/2-power equation is not as sensitive as the rate of average shoot biomass accumulation to water stress. Further analysis indicated that, in each density treatment,the lines of the height versus shoot biomass relationships did not differ significantly between the two water conditions. However, the intercepts of the height versus shoot biomass relationships were greater in the higher-density populations (10 000/m2) than those in the lower-density populations (3 000/m2). These results showed that water deficit did not change plant geometry in this experiment. That is to say, shoot competition for light remains constant at a given biomass, although root competition for water becomes more serious in water deficit conditions. Based on these results and previous reports we propose that, to affect the thinning line slope, changes in symmetric competition are not as efficient as changes in asymmetric competition.