Sequences from the 5' end terminal part of the 28S ribosomal RNA were obtained and compared for two classes Demospongia and Calcarea. Phylogenetic analyses undertaken using different methods showed deep radiations and at least three major groups. In agreement with morphological interpretations, there is a confirmation of the division between the two classes Calcarea and Demospongia; however, rRNA sequences suggest a separation earlier than usually thought. Within Demospongiae our data suggest an early divergence between Haplosclerida and Petrosida on one side and Poecilosclerida, Axinellida, Agelasida and Halichondrida on the other side. Each of these two groups seems to form well separated monophyletic units. These data are clearly in contradiction with the classical taxonomy of demosponges, in particular because the coherence of the sub-class Ceractinomorpha is well accepted. If the respective places of Axinellida and Halichondrida have been a matter of discussion, the presence of Haplosclerida and Poecilosclerida within the same subclass is of general agreement, and recent cladistic analyses place Haplosclerida as sister group to Poecilosclerida. #&The preliminary results suggest that the separation of Haplosclerida-Petrosiida group is a very early radiation. This could be correlated with the fossil record, the middle Cambrian primitive Hazelia, which has a reticulated skeleton of oxeas similar to Haplosclerida-Petrosiida (Finks, 1970).