Three species of sponges, Axinella aruensis, Phakellia sp. and Acanthella cavernosa, were selected to investigate the extent of intragenomic variation (IGV) in the internal transcribed spacers (ITS-1 and ITS-2) of the ribosomal DNA within the order Halichondrida. The IGV was detected in both the spacers and in the three species. At least three non-identical sequences were found in any single individual with the number of polymorphic sites within species ranging from 6 to 77. Uncorrected distance values up to 29% were observed in the ITS-2 of Axinella aruensis. The levels of polymorphism found in this species are the highest reported for Porifera and comparable to those observed in hybrid species of scleractinean corals. Analyses of ITS sequences from multiple clones from individuals of Halichondrida are strongly recommended if these genetic markers are to be used to depict phylogenetic and/or phylogeographic relationships of closely related species and their populations. Data from other nuclear and mitochondrial markers should be used to complement conclusions derived from studies based on ITS sequences.