Light and temperature dependenciee for net photosynthesis and stomatai conductance were generally very similar between foliage on crested wheatgrass (Agropyrou &r&orum (Fisch. ex Link) Scbult.) plants and that on bluebunch wheatgrass (A. spicatum (Pursb) Seribn. and Smith) plants. The simllarlty of these gas exchange characteristics between the 2 bunchgrass species was true for foliage on unclipped plants as well as on putially defoliated plants. However, light and temperature dependencies of senesclng leaf blades that were exserted in late-spring were signifiermntly different for unclipped plants of these 2 species. Photosynthetic rates and stomatal conductances of senescent late-season blades on bluebunch wheatgrass plants were greater than those on crested wheatgrass plants at light intensities greater than 0.8 mm01 pbotons mm2 s-l (photosynthetic photon flux density) and at all foliage temperatures between 180 C and 41° C. These greater photosynthetic rates and stomata1 conductances do not mean tlmt bluebunch wheatgrass tillers gained substantWly more carbon or lost substmrtially more w8ter tb8n crested whe8tgrmss tillers. If both the photosynthetic area composition of tillers and the environmental conditions of the northern Utah study site were considered, carbon gain and water loss for individual bluebunch wheatgrass tillers would be very similar to those for individual crested wbeatgmss tillers despite the significantly different responses to light and temperature during mid-summer.