Many brachyuran crab genera are known to be obligate symbionts on scleractinian corals in shallow tropical seas (e.g. Cymo, Tanaocheles, Domecia, Tetralia, Tetraloides and Trapezia). Most are currently placed in several families and superfamilies, the most prominent being the Trapezioidea, containing Domeciidae, Tetraliidae and Trapeziidae. The distinctness of the domeciids has long been recognised in Domecinae, and finally Domeciidae. Tetraliidae, however, has only recently been recognised when the classical coral obligate family Trapeziidae was split into two families, Trapeziidae sensu stricto and Tetraliidae. Recognition of Tetraliidae, however, has been controversial, and some workers have suggested that it should be regarded as a synonym of Trapeziidae, principally on the basis of carapace morphology and larval features. Using two gene markers, we evaluate the status of Trapezioidea and, in particular, that of Tetraliidae. Our analyses support the distinction between Tetraliidae and Trapeziidae and suggest that crabs have independently colonised hard corals several times.