Preliminary notes are presented on the ecology ofCulex aikenii(Aiken&Rowland), vector of Venezuelan encephalitis (VE) in the middle Chagres river basin of Panama.C. aikeniiprobably represents a complex of at least 2 closely related species with similar habits and both are involved in the natural transmission of VE virus. However, until taxonomic work now in progress is completed the name in its present broad sense will be used. Immature stages of the species are almost always found closely associated with the aquatic floating plant water-lettuce,Pistia stratiotes. Eggs arc laid singly on the leaves. Larvae and pupae remain motionless for hours attached to airpockets under thePistialeaves. WhenPistiaplants are dislodged from their anchorage by flooding, larvae and pupae move down with them and may seed new areas where the plants become tangled and proliferate. These movements ofPistiaseem to serve the double effect of restricting growth of local populations ofC. aikeniiand of serving as one method for dispersal of the species. Females have a broad spectrum of hosts both homeothermic and poikilothermic. They have a slight predilection for mammals over birds. Among the latter, herons are the preferred hosts. The species bites a variety of mammals, but seems to prefer rodents, except where there are large concentrations of domestic mammals, such as cattle. They feed on blood between dusk and dawn with peaks of activity occurring irregularly throughout the night. Females feed preferably on the ground, seldom going up to the canopy of trees in search of hosts. Man is usually bitten very low on the legs, so that people wearing shoes and long pants arc seldom fed on.