Thickspike wheatgrass (Elymus lanceolatus) is an autotetraploid (2n=4x=28), perennial grass characterized by its tolerance to drought and its rhizomatous growth habit, and valued in the Intermountain West for both forage and conservation purposes. Although difficult to establish and generally a poor seed producer, its rhizomatous growth habit allows surviving plants to spread and, in time, produce a thick stand. This study included a group of half-sib (hs) families developed from the polycross of selected individuals from the thickspike wheatgrass cultivars ‘Bannock' and ‘Schwendimar'. Bulked seed from maternal parents formed individual hs families. The experiment began in spring 2004 with the planting of the hs families at two locations: Blue Creek (Box Elder Co., UT) and Nephi (Juab Co., UT) Dryland Research Farms. The experimental design was a randomized complete block design with six complete blocks and each plot comprised of six plants. Data collection occurred in the spring 2005 and consisted of measuring rhizome proliferation at the widest point of each plant. There was a large amount of variation among the hs families for rhizome proliferation with some plants exhibiting rhizome spreads of nearly a meter and others plants exhibiting nearly caespitose growth with no proliferation. Data, to be shown, include estimates of variance components and heritability for this trait and discussion of the suitability of this trait for improvement using traditional selection methodology.