Maintenance of substantial genetic variation for learning ability in many animal populations suggests that learning ability has fitness costs, but there is little empirical evidence for them. In this paper, we demonstrate an evolutionary trade-off between learning ability and competitive ability in Drosophila melanogaster. We show that the evolution of an improved learning ability in replicated experimental fly populations has been consistently associated with a decline of larval competitive ability, compared with replicated control populations. The competitive ability was not affected by crossing of the replicate populations within each selection regime, excluding differential inbreeding as a potential confounding factor. Our results provide evidence for a constitutive fitness cost of learning ability, i.e. one that is paid irrespective of whether or not the learning ability is actually used.