The blood feeding of mosquitoes and black flies from Hereford cattle and ponies treated with commercial formulations of permethrin was evaluated using an animal enclosure trap sample system that allowed comparison of insect blood-feeding levels between treated and nontreated animals. Blood feeding of both Aedes dorsalis Meigen and A. melanimon Dyar from heifers treated with pour-on concentrate and whole body spray treatments was reduced significantly by 79–88% at 4 d posttreatment, with apparent but not significant reductions of 61–68% at 11 d posttreatment. Simulium bivittatum Malloch and S. griseum Coquillett blood feeding was reduced significantly by 96% to >99% at 4 d posttreatment, but apparent reductions of 30–87% at 11 d posttreatment were not significant. Blood feeding of S. bivittatum from ponies treated with a permethrin fly wipe was reduced significantly by 98 and 87% at 1 and 7 d posttreatment, respectively. No evidence of treatment-induced mortality was observed for recently blood-fed female mosquitoes or black flies captured from treated animals and held for 24 h. The potential benefit of using permethrin to protect livestock from insect-transmitted pathogens was estimated with a model based on level of host attack, pathogen infection rate in the vector, and suppression of blood feeding. Suppression of blood feeding by 90% is predicted to prevent the exposure of a host to a pathogen for up to10 d at 1,000 insect feedings per d when the vector population infection rate is one insect per 1,000. If insect feedings are lower (100/d) and the insect infection rate remains at one per 1,000, protection is predicted for 100 d. In contrast, a 90% suppression of blood feeding is predicted to provide protection for less than 1 d at 1,000 feeding per day and a vector infection rate of one insect per 100.