Experiments were done to examine the effect of moisture in the soil on selection of oviposition sites by Aedes vexans. In the laboratory, a device was designed and constructed which presented gravid mosquitoes with a choice of surfaces held at constant moistness. Using this device it was demonstrated that damp surfaces were preferred to either very wet or dry ones. In the field, addition of water to an oviposition site when the area as a whole was dry was sufficient to attract ovipositing mosquitoes. Oviposition was enhanced when suitable levels of moisture in the field were maintained by (1) elevation in relation to water table, (2) by kind and amount of detritus on the soil, (3) by nature and density of canopy, and (4) by frequency and abundance of rainfall.