Seasonal breeding schedules for wild mammals, such as North American river otters (Lontra canadensis), often vary across their range. Previous studies in temperate regions found that river otters typically breed in March and April. Here we report on breeding condition and copulation of otters in June from Yellowstone Lake in Yellowstone National Park. Our findings suggest Yellowstone Lake otters have a relatively long breeding season, extending into June, or their breeding schedule is delayed so that energetically-demanding lactation coincides with spawning runs of Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri).