Consecutive underwater observations carried out in shallow waters of Pontetto (Ligurian Sea) from June to the end of November 2013 revealed the presence of purple areas on the white surface of the asconoid calcareous sponge Clathina coriacea. Histological and ultrastructural investigations performed on the purple areas of the sponge showed the occurrence of a network of hyphae of an indeterminate fungus permeating the sponge tissues and presumably responsible for the unusual colour of the sponge. The hyphae, varying in size and morphology according to their location in the sponge body, are visible on the outermost sponge surface and, after crossing the mesohyl, penetrate inwards into the choanodermal layer, being mainly located along the apical border of the choanocytes. The occurrence of undamaged flagella and microvillar fringes, which are the organelles characterising the choanocyte cells, is consistent with a normal functionality of the sponge tissues, notwithstanding the presence of the fungal hyphae. The fungus develops only in summer because it disappears concomitantly with the approach of autumn. This feature is coherent with the consideration that the sponge acts as a suitable substrate for the developing fungus, which, in turn, does not interfere with the sponge filter-feeding activity.