Interspecific competition among carnivores is an important determinant of predator population dynamics, and it may also affect lower trophic levels. Whereas considerable research has focused on effects of interspecific competition on threatened carnivore populations, here we examine the effects of interspecific competition on the most abundant large carnivore in Africa, the spotted hyena Crocuta crocuta. Specifically, we compared two populations of spotted hyenas to examine the sources of variation in fitness, including effects of competition with lions Panthera leo. Hyenas living in Amboseli National Park and the Masai Mara National Reserve, Kenya, were monitored from 2003 to 2005 and 1988 to 1992, respectively. Females reproduced at significantly higher rates in Amboseli, due to shorter lactation periods, though neither juvenile survival nor litter size differed between populations. Differences in reproduction were estimated to result in 24% greater lifetime reproductive success among Amboseli than among Mara females. We then tested alternative ecological hypotheses to explain these differences in fitness measures between populations. We found little support for hypotheses suggesting that population differences were due to variation in prey availability or dispersion, or to intraspecific competition either within or between hyena groups. Our results were most consistent with the hypothesis that population differences were due to variation in interspecific competition with lions. Hyenas in Amboseli lived at lower lion density, enjoyed higher rates of food intake despite less abundant prey, and scavenged more food from lions than did Mara hyenas. Our results indicate that competition with lions operated primarily via food competition, rather than direct killing, to generate the fitness differences.