Laboratory techniques were developed to screen insecticides for activity as larvicides against stable flies, Stomoxys calcitrans (L.), and horn flies, Haematobia irritans (L.). With these techniques, the susceptibilities of immature, laboratory-reared stable and horn flies to selected insecticides in the larval medium were determined. For stable flies, the LC50 was lower for an IGR, diflubenzuron, and an avermectin, MK-933 (at least 80% of 22,23-dihydro-5-O-demethylavermectin Ala and not more than 20% 22,23-dihydro-5-O-demethyl-25-de(l-methylpropyl)-25-(l-methylethyl)avermectin Ala), than for the other materials tested. For horn flies, the LC50 was lower for methoprene and MK-933 than for the other materials.