We have developed an experimental technique to quantify multiple blood feeding by mosquitoes during a single gonotrophic cycle on conspecific or closely related vertebrate hosts. Rubidium and cesium were detected in mosquitoes that fed on chickens injected with these elements 3 d earlier. Experimentally induced double blood meals were confirmed by the presence of both rubidium and cesium in individual mosquitoes. Multiple blood feeding by Culiseta melanura also was demonstrated in nature using this technique. This was the first simultaneous use of two alkali metals as host blood-markers for the study of mosquito blood-feeding behavior.