Each of 2 herds in Salem County, New Jersey, were divided into 3 groups almost equally susceptible to biting fly attack and providing similar milk production. Two of the groups in each herd were sprayed at weekly intervals with 2 water emulsion formulations: one contained methoxychlor as the active ingredient; the other contained methoxychlor plus butoxypoly-propylene glycol (Crag Fly Repellent). The 3d group was left as an untreated check. Daily observations were made of the number of biting flies on the individual cows and the milk produced. For a period of 5 to 7 weeks, a significant drop in fly numbers (horn, stable, and horse flies) was observable on the treated groups 1-2 days after spraying applications. An increase in milk production was found on 1 farm for 1 day following the methoxychlor formulation and for 2 days following the methoxychlor plus butoxypolypropylene glycol. It was estimated that the increase in milk production more than paid for the cost of fly protection.