In a five-week greenhouse investigation, variations in growth and morphology of monoecious and dioeciousHydrilla[Hydrilla verticillata(L. f.) Royle] were compared over a range of temperatures, 12 to 32 C, on an inorganic sediment and on an organic sediment. For these bio-types, total biomass production was severely restricted at 16 C and below, with thermal optima for growth occurring between 28 and 32 C. Total biomass, shoot number, and shoot length for each biotype were greater on the inorganic sediment than on the organic sediment. At higher temperatures, dioeciousHydrillalengthened more extensively than monoeciousHydrilla; however, the latter produced higher shoot densities and tuber numbers under most experimental conditions. MonoeciousHydrillaappears to be better adapted to moderate temperatures than dioeciousHydrilla, and its potential distribution may include sediments less favorable for growth of the dioecious biotype.