Three experiments were performed to determine whether previous history, or conditioning, of soil influences the final selection of oviposition sites by Aedes vexans. Two of these experiments involved offering gravid mosquitoes a choice of kinds of soil on which to oviposit. Caged mosquitoes in the laboratory and a natural population of mosquitoes in the field preferred to oviposit on artificially moistened soil from oviposition sites rather than on artificially moistened soil from sites that had had no history of flooding. Under natural conditions, a population ceased to oviposit when a major flood covered elevations of the usual oviposition sites. When the water receded, oviposition commenced again. Selection of oviposition sites that are suitably moist and which have been favorably conditioned probably help assure that eggs are located in areas that are likely to flood.