Tree-hole mosquitoes were collected from Western Iowa across Nebraska into Eastern Wyoming and Eastern Colorado to learn more about their distribution and the ecological factors which might influence their distribution. Aedes triseriatus (Say), Aedes hendersoni Cockerell, Anopheles barberi Coquillett, Orthopodomyia signifera (Coquillett), and Orthopodomyia alba Baker were found in Iowa and Nebraska. The occurrence of O. alba in Iowa and Nebraska represented new records for these two states. Only Ae. hendersoni was found in Colorado and Wyoming. For each sample site the following data were recorded: elevation, tree type, diameter of opening into tree hole, height of tree hole above ground, water temperature and pH, and average annual precipitation. The strongest correlation between environmental factor and species distribution appeared to be the influence of precipitation on the distribution of Ae. triseriatus. This mosquito was not found where the average annual precipitation was less than 20 inches.