AFRICAN lions live in complex social groups and show extensive cooperative behaviour1-10. Here we describe a new application of DNA fingerprinting that unequivocally demonstrates the kinship structure of lion 'prides': female companions are always closely related, male companions are either closely related or unrelated, and mating partners are usually unrelated. The variability in relatedness among male coalition partners provides an important opportunity to test for the effects of kinship on cooperative behaviour11. Paternity analysis reveals that male reproductive success becomes increasingly skewed as coalition size increases, and the tendency to form coalitions with non-relatives drops sharply with increasing coalition size. Thus males only act as non-reproductive 'helpers' in coalitions composed of close relatives.