The plasma and fecal concentrations ofeprinomectin weredetermined in cattlefollowing topical administrationat a dose rate of 0.5 mg kg−1. The maximum plasma concentrations of 12.24 ng ml−1 occurred 2 d after administration, and eprinomectin remained detectable in plasma 29 d after administration (0.10 ng ml−1). The maximum dung concentration of 350 ng g−1 was observed 3 d after administration and thereafter for at least 29 d (4 ng g−1). The amount of drug recovered in dung during this period was 20.50% ± 4.31% of the total administered dose. The effects of eprinomectin against the nontarget dung-feeding Diptera Neomyia cornicina was assessed under laboratory conditions. Feces voided by cattle treated with eprinomectin were associated with high larval mortality during the first 12 d after treatment, with null emergence until day 7. The no-observed-effect concentration for N. cornicina was estimated to be close to 7 ± 5 ng g−1.