To determine the systemic efficacy and persistence of a 0.5% ivermectin topical formulation against natural infestation of torsalo, Dermatobia hominis (L., Jr.), larvae in cattle, two trials in Colombia and two trials in Brazil were done. Of 56 infested cattle used in these trials, 28 were treated and 28 served as untreated controls. The formulation was poured along the back of each animal at a rate of 0.5 mg (AI)/kg body weight. Significantly fewer larvae were counted in the treated animals than on controls on days 9 and 10; greater than 99% control resulted. The first reinfesting larvae were observed in a few animals on days 30 and 31.