Plant competition has been recognized as one of the most important factors influencing the structure and function of lake ecosystems.Competition from plants of dissimilar growth form may have profound effects on shallow lakes.An experiment was conducted to investigate the effects of competitive interactions of submersed plants with dissimilar growth forms on the biomass allocations.Hydrilla verticillata and Vallisneria natans were selected and were planted in a single-species monoculture and a mixed-species patt.Results showed that the growth of Ⅴ.natans was significantly affected by the H.verticillata and caused a sharp reduction of biomass,but the root:shoot ratio of Ⅴ.natans was not affected significantly and there was a minimal increase in mixture: while for H.verticillata,the biomass and the root:shoot ratio were not significantly changed by the competitive interactions of Ⅴ.natans,there was minimal increase qf biomass and minimal decrease of the root:shoot ratio.These results may indicate that the phant which candevelop a dense mat or canopy at the water surface would be a stronger competitor relative to the plant that dependsmore on light availability near the sediment.