Brahman-crossbred and English x Exotic feeder heifers were exposed to low (5 per leg), medium (12 per leg), and high (30 per leg) stable fly, Stomoxys calcitrans (L.), population levels to test relative tolerance of these cattle breeds to stable flies. The Brahman-crossbred heifers were tolerant to stable flies only when they were 12-13 mo old. At the same age, the English x Exotic heifers sustained reductions in average daily gain (ADG) at all three stable fly population levels of 0.22 kg/d (11.2%). At 14-15 mo of age, both breeds were affected only by the high stable fly level. The reduction in ADG was approximately 0.16 kg/d (10.6%) for both breeds. Stable fly numbers on the heifers varied with time of day. Both breeds appeared equally attractive to stable flies. With or without stable flies, the Brahman-crossbred heifers consistently gained less weight than the English x Exotic heifers by 0.24 kg/d (15%).