This paper investigates the colonization of concrete tiles by the lichen Caloplaca aurantia in a rural, non-polluted settlement in Israel. The percentage colonization by this crustose Iichen on roof tiles 30, 45, and 60 years old was found to be 2.464 ± 0.732%, 22.972 ± 7.311% and 48.515±6.781% respectively. Scanning Electron Microscopy (SEM) revealed that spherical cells of a unicellular green alga, probably Trebouxia colonize pits on the weathered surface of the very same concrete tiles, as do also the ascopores of C. aurantia. In many of the pits, the ascospores arrange in clusters of 8 units. Fungal hyphae were observed close to ascopores and to the free algal cells.
