Experiments were conducted to determine the role of mosquito-produced attractants and chemicals of environmental origin in the selection of oviposition sites by the western treehole mosquito. Under laboratory conditions, treehole water and laboratory rearing water attractive, but emergence water, larval holding water and water exposed to newly laid eggs were not attractive to ovipositing mosquitoes. The results indicate that decayed organic matter in natural breeding water acts as an oviposition attractant for this species.