Abstract Two submerged aquatic macrophytes, Potamogeton pectinatus L. (sago pondweed) and Myriophyllum exalbescens Fern. (water milfoil) were grown in experimental aquaria in single- and mixed-species cultures in combinations of low and high densities. Biomass of individual plant parts (roots, shoots and tubers) was determined monthly for four months. P. pectinatus root and shoot biomass was greater than M. exalbescens root and shoot biomass, throughout the experiment, at both low and high plant densities. From the relationships between plant densities and their biomass (in the presence and absence of conspecifics and individual plants of the other species), and under specific laboratory conditions, we found that: (1) intraspecific competition, as detected from the decrease in growth rate with increased plant density, was present; (2) M. exalbescens did not exert an interspecific competitive effect on P. pectinatus; (3) the interspecific competitive effect of P. pectinatus on M. exalbescens was stronger in inhibiting plant growth than that of M. exalbescens on itself; (4) increased plant density resulted in smaller P. pectinatus roots, shoots and tubers; (5) shoots of M. exalbescens, but not roots, were affected by increased plant density.