Nebraska is one of the states in which the tree-hole mosquito Aedes triseriatus (Say) reaches its western limit; it is found only in the eastern half of the state. Ae. hendersoni is found throughout the state. Samples of these 2 mosquitoes were collected from extreme western Iowa across Nebraska into the extreme eastern portions of Wyoming and Colorado. Careful study was made of 24 morphological traits of the 4th instar larvae to characterize these 2 species in this part of their geographical range. Seven of the 24 traits were of taxonomic value. A spot check of 4 additional traits revealed that all of those appeared to be of value making a total of 11 diagnostic characteristics.