The rare aquatic macrophyte Luronium natans can be encountered with two growth forms: as a bottom-dwelling plant with a rosette of linear leaves, or as a nymphaeid plant with long-petioled oval floating leaves. The change of growth form corresponds to a change in size class (small and submerged vs. tall and canopy-forming). The present study compares through one growing season floating leaf production and patterns of biomass allocation in Luronium ramets from a natural, moderately nutrient-rich habitat and from nutrient-rich transplantation sites. We ask whether differences in gross floating leaf production result from differences in ramet vigour while allocation patterns remain stable, or whether Luronium is able to allocate biomass plastically, and thus to change its growth form with differences in habitat. The study shows that both leaf types were produced in all habitats since the mid-growing season (June), but that biomass allocation to them varied. Floating leaves were dominant at high nutrient levels, whereas at the moderate nutrient level most leaf biomass was found in the rosette. We suggest that Luronium modifies its biomass allocation in a pattern that corresponds to the outlines of Tilman's (1988) mechanistic model of optimal biomass allocation on a light:nutrient gradient. The demonstrated morphological plasticity may enable Luronium to maintain itself in a large range of habitats, but it raises new questions about the reasons for the species' rarity.