Homoscleromorpha were recently proposed as fourth class of sponges. But classification of homoscleromorph species is often problematic due to the lack of diagnostic characters, especially for the genus Oscarella that lacks a skeleton. Recently, bacterial ultrastructure was suggested as an additional cytological marker. Using molecular techniques we confirm distinct microbial communities in five Mediterranean Oscarella species. The bacterial profiles are characteristic of low microbial abundance (LMA) sponges. This was further confirmed by analysis of a 16S rRNA clone library from O. lobularis that yielded a low phylum-level diversity with dominance of Alphaproteobacteria. Bacterial communities in O. lobularis were very similar among different individuals (collected at the same site and time), five different color morphotypes, and specimen from different depths and locations indicating a very stable association. These results not only confirm bacteria as useful markers for Oscarella classification but also pro