The phylogenetic position of the platyhelminths within the metazoan tree is examined using two independent sets of molecular characters, the evolution of 18S ribosomal RNA sequences and the diversity of the genes belonging to the HOX cluster. Among the various hypotheses that have been considered by zoologists, a position of the platyhelminths within the protostomes, related to the phyla with typical spiral cleavage, appears to be favoured when taking into account all separate lines of evidence. It is in conflict with the traditional hypothesis of an early emergence at the base of the bilaterally symmetrical animals. This relatively late emergence is compatible with the old idea that flatworms are derived from a coelomate ancestor. New evidence from the sequences of Hox genes suggests that the duplicated genes Ultrabithorax/abdominal-A constitute a genetic synapomorphy of the whole protostome clade.