Cannibalizing a related individual can reduce the inclusive fitness of the cannibal. Hence, mechanisms that allow a tadpole to recognize and modify its behavior toward kin may reduce the inclusive fitness costs of cannibalism. Alternatively, ecological factors may cause preferential treatment of kin to be too costly to be favored by selection. We tested these two predictions in the Green Poison Frog, Dendrobates auratus. The effect of kinship on larval cannibalism was examined through a series of kin-discrimination trials. The behavior of large tadpoles was observed when presented with two small, tethered tadpoles, one a clutchmate and one an unrelated tadpole. In these simultaneous presentation tests, tadpoles displayed a significant preference for attacking kin. In a series of timed trials, pairs of unequally sized tadpoles were placed together in containers. The majority (70%) of large tadpoles took less than24 hr to consume the small tadpole. Kinship did not affect the survival time of the small tadpole. Our results are consistent with observations that D. auratus is an indiscriminate predator. As conspecifics may be serious competitors, their swift elimination would be an advantage, particularly in the small, nutrient-poor poolsused by this species.