Crambe crambe is a marine sponge that produces high concentrations of the pharmacologically significant pentacyclic guanidine alkaloids (PGAs), Crambescines and Crambescidines. Although bio-mimetic chemical synthesis of PGAs suggests involvement of microorganisms in their biosynthesis, there are conflicting reports on whether bacteria are associated with this sponge or not. Using 16S rRNA gene pyrosequencing we show that the associated bacterial community of C. crambe is dominated by a single bacterial species affiliated to the Betaproteobacteria. Microscopy analysis of sponge tissue sections using a specific probe and in situ hybridization confirmed its dominance in the sponge mesohyl and a single microbial morphology was observed by transmission electron microscopy. If confirmed the presence of a simple bacteria community in C. crambe makes this association a very pertinent model to study sponge-bacteria interactions and should allow further research into the possible implication of bacteria in PGA biosynthesis.