We assess the potential for American black bears (Ursus americanus) to limit the growth of colonizing or severely reduced grizzly bear (Ursus arctos) populations. Managers are faced with the challenge of increasing the size of small (N < 75) grizzly bear populations in the North Cascade, Selkirk, Cabinet–Yaak, and Bitterroot recovery areas of the USA and Canada. These populations are mainly limited by human-caused mortality. However, other factors such as competition from black bears could impose additional constraints. Brown and grizzly bears and American black bears evolved separately until about 13,000 years ago and, as a probable consequence, they can have substantial diet overlap. Where meat and roots are available, grizzly bears consume more of these foods than do black bears. Where fleshy fruits and succulent forbs are the primary high quality bear foods, as in the North Cascade, Selkirk, and Cabinet–Yaak ecosystems, dietary overlap between grizzly and black bears can be almost complete. Largely because they are smaller, black bears can exist at roughly 10 times the density of grizzly bears, use ranges that are, on average, four-fifths smaller, and are more efficient than grizzly bears at using low densities of small berries. We postulate that the primary impact of black bears on grizzly bears is through reduced reproduction and recruitment caused by exploitation competition, despite the documented ability of most grizzly bears to dominate most black bears during physical confrontations. Such an effect would be greatest in areas where both species rely on berries and forbs, where grizzly bear populations have been extirpated, substantially reduced, or are absent but within dispersal distance, and where black bear populations are comparatively robust. On this basis we postulate that exploitation competition by resident black bears, together with mortality caused by Native Americans, slowed or even curbed the invasion of grizzly bears east across North America during the late Pleistocene and early Holocene. We also postulate that grizzly bears are absent on some coastal islands within dispersal distance of robust grizzly bear populations because of competitive exclusion by black bears.