The micro-erosion patterns of some sponge species belonging to the boring genera Cliona and Aka have been compared. The scanning electron microscope analysis of the excavations of species of the two genera sharing the same substratum clearly indicates that the scars produced on the walls of the boring chambers are different. The pits of Cliona are regular, almost circular and smooth; those of Aka are more irregular and show a multi-layered concentric pattern. These differences constitute a successful means of distinguishing the bio-erosion agents both in recent and in fossil substrata.