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We have employed electronmicroscopical methods (SEM, TEM) to document the microbial community associated with the marine sponge Aplysina cavernicola (formerly Verongia cavernicola, class Demospongiae). Five dominant bacterial types were identified, three of which resemble the morphotypes originally described by Vacelet (1975). One bacterial type possesses morphological properties that are characteristic of the genus Planctomyces. In addition, morphologically uniform bacteria which reside inside the nuclei of host cells were observed. Using in situ hybridization with fluorescently labelled rRNA probes directed against known bacterial groups, the phylogenetic affiliation of the mesohyl bacteria was assessed. It could be shown that the vast majority of mesohyl bacteria belongs to the domain Bacteria with a low GC content. Among the Bacteria, the delta-Proteobacteria were most abundant …