This paper provides Wrst information on organlike bacterial aggregates in the tentacles of the sea anemone Metridium senile. The specimens were collected from waters near Helgoland (German Bight, North Sea) and the Orkney Islands. Tentacles were prepared for morphological inspection by light and scanning electron microscopy as well as for the phylogenetic analysis of endocytic bacteria. Bacterial aggregates are located in caverns of the tentacles' epidermis. The aggregates are enwrapped in thin envelopes, which contain coccoid and/or rod-shaped tightly packed bacteria of diVerent division states. Most of the bacterial cells are connected by Wne Wlamentous structures. The phylogenetic determination is based on the sequence data of the 16S rDNA derived from tentacle material. Sequence analysis revealed three diVerent subgroups of intratentacular proteobacteria. The dominant band, detected in all of the samples tested, showed a close relationship (98%) to a gram-negative Endozoicimonas elysicola. Two bands, only detected in tentacles of M. senile from Helgoland were assigned to Pseudomonas saccherophilia (99%), a knallgas bacterium, and to Ralstonia pickettii (100%). The bacteria represent a speciWc bacterial community. Their DGGE proWles do not correspond to the proWles of the planktonic bacteria generated from seawater close to the habitats of the anemones. The allocation of DNA sequences to the diVerent morphotypes, their isolation, culturing and the elucidation of the physiological functions of intratentacular bacteria are in progress.