In this paper, the distribution of sponge Stelletta tenui associated microorganisms was investigated by transmission electron microscope and the characteristics were discussed on the basis of sponge structure and nutrition ingestion. It was proved that there are abundant microorganisms in sponge mesohyl, sponge cells and the inner cavity except for the sponge bone, in which sponge mesohyl is the main site for symbiotic microorganisms. It was suggested that the microbial distribution characteristics are due to the cavity structure and the nutrition ingestion of sponge by water filtration to some extent, which is useful for us to understand the mechanism of microbial accumulation in sponge and the relationship between sponge associated microorganisms and the w ater environment in the future.