Experiments of plant growth responses to different CO₂ concentrations and temperatures were conducted in growth chambers to explore the interactive effects of atmospheric CO₂ enrichment and temperature on the growth and dry matter allocation of dioecious Hydrilla [Hydrilla verticillata (L.f.) Royle]. Hydrilla plants were exposed to two atmospheric CO₂ concentrations (350 and 700 ppm) and three temperatures (15, 25 and 32°C) under a 12-hr photoperiod for about 2 months. The plant growth analysis showed that elevated CO₂ appeared to enhance the growth of Hydrilla, and that the percentage of the enhancement is strongly temperature-dependent. Maximum biomass production was achieved at 700 ppm CO₂ and 32°C. At 15°C, the total dry matter production was increased about 27% by doubling CO₂, due to a 26% enhancement of leaf biomass, a 34% enhancement of stem biomass and 16% enhancement of root biomass. At 25°C, the dry matter production was increased about 46% by doubling CO₂, due to a 29% enhancement of leaf biomass, a 27% enhancement of stem biomass and 40% enhancement of root biomass. At 32°C, however, the percentage of the enhancement of total dry matter production by doubling CO₂ was only about 7%. The dry matter allocation among different plant parts was influenced by temperature but not by elevated CO₂ concentration.