Abstract The Green River greenthread (Thelesperma caespitosum) was first discovered in 1947, but was not described as a new species until 1990 (Dorn 1990). It is closely related to Thelesperma pubescensand other taxa in the "T. subnudum" complex, differing primarily in having glabrate leaf blades and rayless flower heads. T. caespitosum is known from 2 locations near the city of Green River, Wyoming (Sweetwater County) and 2 mainsites south of Duchesne, Utah (Duchesne County). Recently, 3 additional colonieshave been discovered in southern Duchesne County consisting of T. caespitosum, T. pubescens, and morphologically intermediate plants. Biosystematic studies by Hansen (1998) suggest that T. caespitosum should be considered a variety of T. pubescens. In Wyoming, T. caespitosumis restricted to sparsely vegetated cushion plant and bunchgrass communities with low shrub cover on ridgetops and upper slopes of bleached, limey-slate derived from the Wilkins Peakmember of the Green River Formation. Utah populations occur on similar outcrops of the Uinta or Green River formations in openings within pinyon pine and mountain mahogany. Surveys in Wyoming in 1997-1998 resulted in the discovery of 3 additional subpopulations on the west side of the Green River. The total state population is currently estimated at 26,500-31,500,a figure that is comparable to estimates made in 1994 (Fertig 1995). Most populations in Utah have not been resurveyed in recent years, although at least one "mixed" T. pubescens-T. caespitosumcontained thousands to tens of thousands of plants in 1995 (S. Goodrich, personal communication). Green River greenthread is threatened by habitat damage from off-road vehicle recreation, disturbance associated with mineral exploration and development, and urban expansion. Most known populations are on public or tribal lands managed for multiple use andrecreation and none currently receive formal protection. Populations on BLM lands in Wyoming could be eligible for designation as a "Special Status Plant" ACEC and the species should be managed as “Sensitive” on public lands. Listing as Threatened or Endangered may be warranted in the future if current public land management fails to prevent significant population declines.