This research was conducted to determine the effects of temperature and competition on seedling root and shoot growth of cheatgrass (Bromus tectorum L.) and 5 collections of Idaho fescue (Festuca idahoensis. Elmer). For the temperature study, seedlings were grown in environmental chambers with 16 hours day/8 hours night, at temperatures of 5, 10 and l5°C. Root depth was weekly recorded during 9 weeks growth period. Seedlings were destructively harvested after 63 days and evaluated for root and shoot characteristics. At all temperatures, cheatgrass exhibited greater above and below-ground biomass, root depth, total root length, daily root depth rate and shoot height than Idaho fescue. Seedlings of the 5 Idaho fescue collections produced fewer roots and grew slower than seedlings of cheatgrass. As a group, the Idaho fescue collections showed similar root and shoot growth and produced more tillers than cheatgrass, except at 5°C. As temperature increased from 10 to 15 °C, shoot growth of Idaho fescue increased while that of cheatgrass was similar. For the competition study, seedlings were grown in pots in a greenhouse with planting ratios of Idaho fescue to cheatgrass of 1:0, 1:5 and 1:10, and under 2 watering regimes. Seedlings of Idaho fescue were established from transplants grown two months ahead of cheatgrass. Shoot height, tiller and leaf number were weekly recorded. Seedlings were harvested after 49 days and evaluated for aboveground dry weight. Competition from cheatgrass depleted soil moisture and reduced growth of Idaho fescue. Cheatgrass had greater aboveground dry weight and shoot height than Idaho fescue. However, Idaho fescue had greater tiller and leaf number than cheatgrass. In addition, when competing with cheatgrass, remnant Idaho fescue plants were less adversely affected in aboveground dry weight and shoot height than Idaho fescue plants from pristine areas. These results indicated that remnant Idaho fescue plants may be a different ecotype occurring in response to competition and may better tolerate the effect of the interference from cheatgrass. This information may serve as a basis for reestablishment of Idaho fescue in Central Oregon rangelands. / Graduation date: 1993