A previously unknown association between a luminous bacterium, Vibrio harveyi, and a benthic hydrozoan, Aglaophenia octodonta, is described. Aglaophenia hydrocladia showed a clear fluorescence in the folds along the hydrocaulus and at the base of the hydrotheca, suggesting the presence of luminous bacteria. This hypothesis was confirmed by isolation of luminous bacteria from Aglaophenia homogenates. Phenotypic characterization of bacterial isolates was performed by several morphological, biochemical, and cultural tests, completed with 16S rDNA sequence analysis. All the isolates were referred to a single species: V. harveyi. The association between V. harveyi and A. octodonta has epidemiological as well as ecological significance. Therefore, A. octodonta may function as habitat "islands" providing a unique set of environmental conditions for luminous bacteria colonization, quite different from those already recorded from the plankton for other Vibrio species.