The habitat differentiation among the epiphytic hydroids at different levels on Posidonia oceanica was examined by use of two statistical techniques. The first technique identified associations and segregations between leaf bundles and within bundles between leaves of different age. The second technique examined selected pairs of hydroid species. Most of the segregations and associations at the level of the bundles are possibly because of similar or dissimilar depth distributions. In two cases, Sertularia perpusilla and Plurnularia obliqua, and Sertularia perpusilla and Campanularia asymmetrica the possibility of competitive interactions was indicated by their segregation between bundles (not associated with different depth distributions). Sertularia perpusilla and Plumularia obliqua also showed segregation between different faces of the leaves.