Sampling of a grasshopper (Acrididae) population weekly during the summer months of 1975, 1976, and 1977 showed that hatching began when the soil temperature (one cm below the surface) was above 15.6°C for a total of at least 200 h and when the degree hours (number of degrees each hour that the soil temperature exceeded 15.6°C) totaled 2466. Grasshopper density decreased from 1975–77. In 1975Melanoplus sanguinipes(F.) totaled 53% of the number collected; in 1976 and 1977,Amphitornus coloradus(Thomas) was most abundant and comprised 23% and 30%, respectively, of the population. Grasshoppers became active at surface temperatures (5 cm above ground) of 13°–16°C, and feeding began at 21°C. Three-yr averages showed the surface temperatures remained above 21°C for 12 h each day in July, 10 h in August, and 8 h in Sept. Seasonal growth, including seed set, of 3 grass species (western wheatgrass—needleandthread—prairie junegrass) was mostly completed before grasshoppers developed to the damaging stage (4th and 5th instars and adults). Clipping studies showed that grass and forb production decreased from 1473 kg/ha in 1975 to 503 kg/ha in 1977 and was directly correlated with spring and summer precipitation. Forage lost to grasshoppers was estimated at 235 kg/ha in 1975, 226 kg/ha in 1976, and none in 1977.