We examined the effects of water temperature on growth of Zizania texana (Texas wild rice), with a distribution in the wild restricted to the upper 4.9 km of the San Marcos River, Hays County, Texas. Plants were grown in tanks randomly assigned to 1 of 3 water temperature treatments: 15.5 ± 1.5, 22.5 ± 1.5, and 28.5 ± 1.5°C for 10 weeks during fall 2001. Leaf number, leaf length, and number of reproductive culms were recorded weekly for each plant. Biomass was recorded after 10 weeks. The effects of temperature, week, and their interaction were significant for leaf number and leaf length. Greatest mean leaf number occurred at week 10 for plants grown in 15.5°C water (27.5 ± 6.3); week 8 for 22.5°C water (63.2 ± 4.4); and week 9 for 28.5°C water (42.4 ± 5.5). Leaf length decreased in 15.5°C water from 37.6 ± 3.4 to 32.9 ± 3.3 cm, but increased in 22.5°C water from 41.0 ± 0.6 to 62.0 ± 2.6 cm and from 41.8 ± 0.8 to 60.3 ± 2.7 cm in 28.5°C water. Plants grown in 15.5°C water did not produce reproductive structures, and plants grown in 28.5°C water produced more culms per plant (3.3 ± 0.31) compared with plants grown in 22.5°C water (0.57 ± 1.07). Temperature influenced leaf, culm, root, and total biomass. Total biomass was less in 15.5°C water. Leaf biomass differed among all treatments. Plants in 28.5°C water allocated more biomass to reproductive organs compared with other treatments. Data from temperature recorders, placed at 4 locations downstream from the current range of Z. texana, showed that, although the mean temperature remained fairly constant among sites, the range of temperatures recorded increased with distance downstream.