The relationships between grasshopper densities and climatic variables in southern Idaho were examined using historical grasshopper survey records from 1950 to 1980. Mean annual grasshopper densities were calculated from grasshopper counts made within a 20-km radius of each of 26 weather stations spanning the Snake River plains and adjacent areas. The mean annual densities for each location were averaged to yield 27-yr mean grasshopper densities. Maximumr2improvement regression techniques were used to select average weather factors that were correlated with grasshopper densities. The average November precipitation and mean April temperature proved to be the best indicators of the mean grasshopper density for a location, accounting for 92% of the variation in the 27-yr mean densities. The relationships of temperature and precipitation to annual grasshopper population fluctuations were analyzed at 15 locations. Most of the mean monthly temperature variables included in the regression equations were positively correlated with grasshopper densities, except for November, December, and January mean temperatures, which were negatively correlated with grasshopper densities in the following year. Precipitation variables included in the regression models were positively correlated with grasshopper densities at nearly all locations. Precipitation appears to be an important factor driving grasshopper population dynamics in southern Idaho, possibly by its effect on vegetation quality and quantity.