Adult horn flies, Haematobia irritans (L.), and stable flies, Stomoxys calcitrans (L.), were fed on a blood diet that was fortified by the addition of known quantities of ivermectin [Merck MK933, at least 80% of 22,23-dihydro-5- O -demethylavermectin A10 and not more than 20% 22,23-dihydro-5- O -demethyl-25-de(1-methylpropyl)-25-(1-methylethyl)avermectin A12], came from heifers injected with the therapeutic dosage of ivermectin, or came from steers treated with a sustained-release implant containing ivermectin. The effect of ivermectin on flies was dependent upon species and sex of flies and dosage and length of time the flies fed on blood. Horn flies were more susceptible than stable flies and males of both species were more susceptible than females. Mortality increased with length of time flies fed on blood. In addition to mortality, ivermectin in the blood diet reduced number and hatch of eggs. The LC50 (and 95% CL) was 3.2 ng/ml (2.9–3.4) for male horn flies and 6.6 ng/ml (6.2–7.0) for female horn flies fed on treated blood for 88 h. For male stable flies fed on treated blood for 72 h, the LC50 was 37.5 ng/ml (26.0–50.2) of blood. The value for female stable flies was outside the range of dosages tested.