The relationships between three species of thecate hydrozoans, Sertularella crassicaulis, Sertularella ellisii and Aglaophenia tubiformis with three species of encrusting Corallinales, Pneophyllum fragile, Melobesia membranacea and Hydrolithon cf. farinosum from different locations in the western Mediterranean are described. In Aglaophenia tubiformis, algae were observed on the hydrotheca, while in Sertularella spp. they grew on the stem, the branches and the hydrothecae which became completely covered by encrusting, calcified thalli. The polyps were living in the covered theca. The hydroid species hosting Corallinales were a small fraction of the entire assemblages and this evidence suggests that hydroids cannot be considered as a simple, inert substrate. We hypothesize that this specificity could be explained by a different molecular composition of hydroid exoskeletons, and/or production of secondary metabolites that affect organism settlement. Moreover, perisarcal areas close to hydranths were more abundantly covered by algae, probably due to nutrient emission generated by the metabolism of the hydroid.