To test the competing hypotheses of polyphyly and monophyly of "sclerosponges," sequences from the 5' end of 28S ribosomal RNA were obtained for Astrosclera willeyana, Acanthochaetetes wellsi, and six other demosponge species. Phylogenetic relationships deduced from parsimony and neighbor-joining analyses suggest that these sclerosponges belong to two different orders of Demospongiae: Astrosclera willeyana, being closely related to the Agelasidae, belongs to the Agelasida, Acanthochaetetes wellsi, being closely related to the Spirastrellidae, belongs to the Hadromerida. These results contradict the hypothesis that sclerosponges are monophyletic and imply that a massive calcareous skeleton has evolved independently in several lineages of sponges.