Observations on the mating and parental behaviour of the green poison-dart frog were used to test two hypotheses for the function of female-female competition in this species. The hypothesis that females compete to monopolize as many mates as possible was not supported because: (1) male territoriality was associated with competition for females; (2) intra-female competition for mates was not more common or intense than intra-male competition; (3) females were more selective about mating than were males; (4) estimated offspring production by individual females did not exceed the parental capacity of a single male. The hypothesis that females compete to monopolize the parental care of particular males was consistent with observations that: (1) females competed for males, but were more selective about mating than were males; (2) females remained near particular males and attempted to keep other females from mating with those males; (3) females may have used courtship to prevent males from mating with other females. Both hypotheses were consistent with evidence that: (1) females are large compared with males in D. auratus, relative to closely related species with low male parental investment; (2) females destroyed the eggs of other females when they located them.