The effect of population densities on the growth rate and metal (nickel) accumulation capacity of Lemna gibba (Lemnaceae) was studied under laboratory conditions. At high population densities (800 or 1600 seedling per pot), we observed decreasing growth rate with increasing L. gibba density. Especially, at a density of 1600 seedling a pot the net growth rate became negative. This lower growth rate may relate to lower local temperatures and nutrients within L. gibba mats. Biomass production was inhibited by an increase in plant density. Additionally, high population densities caused decreasing nickel accumulation by L. gibba. In this study, a simple model was created according to data obtained from this study. The model has suggested that crowding is an important factor in limitation of growth of L. gibba. (c) 2006 Elsevier Ltd. All rights reserved.