We provided 2 social groups of yellow-bellied marmots (Marmota flaviventris) with 2 types of supplemental feed (high and low protein) to test hypotheses about effects of food abundance and quality on juvenile growth rates and survival. Both supplemented litters and reference litters ceased to gain body mass by 2 weeks before hibernation even though supplemented litters had access to supplemental food. Body mass at weaning did not differ significantly between the 2 groups, but supplemented young had significantly higher growth rates (21.4–48.7 g/day) than reference young (24.1–33.3 g/day) and therefore had higher body mass late in the season. Juveniles that received high-protein supplements grew faster than the low-protein group. The addition of high-protein feed did not increase survival; however, juveniles that received low-protein feed were more likely to survive. Juveniles with higher body mass were not more likely to survive. Marmots must reach a critical body mass to survive hibernation and do not appear to benefit from mass gain beyond the critical body mass.