Height-growth of individual plants of Chenopodium album in even-aged monospecific stands with initial densities of 400, 800, and 3600 plants m-2 was followed from emergence to fruit maturation to elucidate the factors responsible for size hierarchy formation. Many individuals stopped growing rather abruptly at relatively early stages in the vegetative phase and some of them eventually died. The other plants grew until fruit maturation and attained large final heights. Depending on whether they terminated or continued height-growth during the vegetative phase, the plants were called 'lower' and 'upper' plants. The numbers of the 'upper' plants per unit area were about 100 plants m-2 irrespective of the initial plant densities, and thus the number of 'lower' plants increased with the increase in the plant density. However, there was an upper limit to the number of the surviving 'lower' plants (400-700 plants m-2). The shapes of the final frequency distributions of plant height were largely determined by the ratio of the number of 'upper' plants to that of surviving 'lower' plants, and were bimodal for the 400 plants m-2 plot and L-shaped for the 800 and 3600 plants m-2 plots. From these, it is concluded that the segregation of the plants into 'upper' and 'lower' plants is an essential factor responsible for the size hierarchy formation. The present results also indicate that both bimodal and L-shaped distributions can result from one-sided competition for light, depending on the initial plant densities.