A morphological study of bacteria associated with the "Pompei worm,"Alvinella pompejana, collected from East Pacific rise hydrothermal vents, revealed four types of epidermal associations on the dorsal part of the animals: various single cells distributed on the animal tegument; clump-like associations located in the intertegumentary spaces; and filamentous bacteria associated with epidermal expansions, or inserted on the posterior parapodia. The bacterial morphologies were illustrated by SEM and TEM pictures and compared to bacteria previously described. The functional significance of these associations is discussed.