The Chthamaloidea (Balanomorpha) present the most plesiomorphic characters in shell plates and cirri, mouthparts, and oral cone within the acorn barnacles (Thoracica: Sessilia). Due to their importance in understanding both the origin and diversification of the Balanomorpha, the evolution of the Chthamaloidea has been debated since Darwin's seminal monographs. Theories of morphological and ontogenetic evolution suggest that the group could have evolved multiple times from pedunculated relatives and that shell plate number diminished gradually (8¿6¿4) from an ancestral state with eight wall plates surrounded by whorls of small imbricating plates; but this hypothesis has never been subjected to a rigorous phylogenetic test. Here we used multilocus sequence data and extensive taxon sampling to build a comprehensive phylogeny of the Chthamaloidea as a basis for understanding their morphological evolution. Our maximum likelihood and Bayesian analyses separate the Catophragmidae (eight shell plates and imbricating plates) from the Chthamalidae (8-4 shell plates and no imbricating plates), but do no support a gradual reduction in shell plates (8¿6¿4). This suggests that evolution at the base of the Balanomorpha involved a considerable amount of homoplasy.