A simple olfactometer was constructed for studying physical and chemical factors attracting Aedes aegypti (L.) man. A stream of 1% CO2 in air activated and attracted the mosquitoes. It was more attractive at 60% relative humidity (rh) than at the other rh tested (26%, 42%, 76% and 90%). Mosquitoes were deterred from high rh (76% and 90%), but thirsty mosquitoes were attracted to high rh (76%). Mosquitoes were attracted to convection heat (34°C) if the air was humid (40% rh) and to a warm air stream if the air was humid (60% rh), but not if the air was dry. The attraction of mosquitoes to lactic acid was confirmed. When passed over a human forearm, a 1% CO2 stream was more attractive than a stream of air, though both attracted mosquitoes. One percent CO2 at 34°C and 60% rh was as attractive as 1% CO2 passing over the forearm. A 5% stream of CO2 passed over glass beads upon which emanation of the forearm had been trapped attracted mosquitoes from 1–5 min., depending on how much emanation had been trapped. The causes for discrepancy in the results of various investigators on the responses of mosquitoes to CO2 and humidity are discussed.