The efficacy of abamectin 1%, when injected subcutaneously in cattle at a dose of 200 micrograms/kg body weight, against the larval stages (grubs) of the fly Dermatobia hominis was evaluated in two trials in endemic areas of Brazil and Argentina. Eighteen Holstein x Brahman castrated males and 16 Brahman-cross with natural infestations were used. Larvae were counted by instar in situ on both sides of each animal before treatment, and were expressed, identified as to stage and classified as live or dead 10 days after treatment. Further larval counts were made periodically until day 79 to evaluate the degree of reinfestation and the stage of larval development. Reinfestation was first detected in the abamectin-treated cattle on day 44. Live larvae were found on 6-8 (Argentina) and on all (Brazil) controls at each post-treatment examination. The difference in numbers of live larvae between treatment groups was statistically significant (P < 0.05) at all post-treatment examinations. These data show that abamectin at a dose of 200 micrograms/kg body weight is highly effective in the treatment and control of established parasitic stages of D. hominis in cattle. No adverse reactions were observed in any of the treated animals.