Abstract:The distribution of Hematobia irritans is not homogeneous in cattle populations. Generally, between 15% and 30% of bovines carry more than 50% of the flies, and higher infestations occur in bulls. In this study, the effect of treating the bulls with ear tags containing 40% diazinon on the infestation of flies on the herd during the breeding season was evaluated. Four groups of cattle were made, each containing one bull and 35 Braford cows. Groups 1 and 2 were on pastures in highlands. Groups 3 and 4 were on pastures in lowlands.