Researchers at Oklahoma State University have successfully transformed the ‘Bobwhite' cultivar of spring wheat with the mannitol-1-dehydrogenase (mtlD) gene from E. coli. The transformed lines accumulate the sugar alcohol mannitol either in the cytosol of the cells (lines pTA2-1 and pTA2-2) or in the chloroplasts (lines pTA5-1 and pTA5-2). Although not naturally present in wheat, mannitol is found in several other plant species, and is thought to aid in drought tolerance by increasing the solute potential of cells and/or by scavenging reactive oxygen species (ROS) generated during periods of water stress (Abebe et al., 2003. Plant Physiology 131: 1748-1755). We planted the four transgenic lines along with the non-transformed parent line and five check varieties in fully irrigated and water-stressed conditions in Fort Collins, Colorado in the 2005 growing season. Although preliminary greenhouse experiments had shown increased drought tolerance in the transformed lines, this was the first field trial of these lines. Drought tolerance was evaluated by several parameters, including relative water content at jointing, heading, and grain-filling; and by measuring total biomass, harvest index, grain yield, 200-kernel weight, and carbon isotope ratios of leaves and grain. Leaf samples were also taken at anthesis and two weeks post-anthesis for measurement of mannitol content and amount of ROS scavenging. Results will be presented and discussed.