During a 2-year study (1969-70), Psorophora confinnis (Lynch-Arribalzaga), Anopheles quadrimaculatus Say, and A. crucians Wiedemann populations caused statistically significant and economically damaging reductions in the average daily gain of feedlot steers. Steers on a high-energy-level ration (20% roughage) and steers on a low-energy-level ration (60% roughage) that were protected from mosquito attack gained significantly more ( P ‘0.01) during certain periods than steers on the same rations exposed to mosquito attack in 1969 and 1970. During these periods of loss in weight-gain efficiency, 6.8-114.7 blood-fed mosquitoes/ft2 wall were observed in the unscreened barn from 30 to 120 min after official sunset. Steers on the low-energy-level ration which were protected from mosquito attack gained in 1969 18.2 lb/head and sold for $5.78/head and in 1970 gained 45.5 lb/head and sold for $13.12/head more than unprotected steers on the same ration. Mosquito attack did not reduce the economic value of the steers on the high-energy level ration in 1969 and reduced their value only by $0.35/head in 1970 No overall significant difference was found between the average daily feed intake of the steers which were exposed to mosquito attack and those protected by screen wire. Larger numbers of blood-fed mosquitoes were observed in the barn 60, 90, and 120 min after official sunset than at 30 minutes after sunset ( P ‘0.01). However, the rate of increase from 90 to 120 minutes after official sunset was much less than that observed during the 1st 90 minutes.