Observations were made on the rates at which mosquitoes (Aedes spp.) and stable flies (Stomoxys calcitrans (L.)) in an experimental chamber landed on a subject wearing a lightweight rubber diving suit which prevented the release of water vapor, carbon dioxide, and odors into the environment. In some tests dry or moist white suits were worn over the diving suit, and in half the tests carbon dioxide was discharged over the subject. and a dummy, which was used to determine the standard landing rate. Aede aegypti (L.) was very active even when no one was in the room and responded readily to the subject in the diving suit. Exposition of the hands increased the number of landings, whereas exposition of the face either failed to increase or actually decreased the landing rate. Aedes taeniorhynchus (Wiedemann) was less active when no one was in the room. The number of landings on the dummy was always low, and few mosquitoes landed on the subject unless the hands and face were exposed or carbon dioxide was released. Stomoxys calcitrans was moderately active, but responded to the subject only when the face of the subject was exposed or when a soiled suit was worn over the diving suit. Carbon dioxide had little stimulating effect on this species.