Insects are often immobilized to facilitate handling during experiments. Of many methods used, one of the earliest was chilling the insects to a point below their activity threshold. As an alternative to chilling, anesthetics such as chloroform and ether have been used. Busvine (1957) pointed out that anesthetics of this type may affect general metabolism and that their fat solvent vapors may disrupt the epicuticular layer of the cuticle. In recent years CO2 has been commonly used. Busvine (1957) has summarized some disadvantages of this method. In this paper we present data on the susceptibility of stable flies, Stomoxys calcitrans (L.), and horn flies, Haematobia irritans (L.), to immobilization by exposure to pure CO2, to a 1:1 mixture of CO2 and air, and to chilling at 4°C.