The phytosociology of lichens colonizing granitic churches and dolmens in Galicia (N.W. Spain) and Alentejo (central Portugal) was studied so as to identify (a) a group of species whose frequency on these structures would make them potentially useful as test species for the evaluation of biocides used for conservation, (b) environmental factors likely to control species distribution, and (c) species that may be useful as bioindicators of environmental factors in this kind of construction in the region studied. Chief among the factors correlating with species distribution is the class of structure (church or dolmen), which is attributed to the probable influence of this factor on environmental parameters affected by structural complexity, the presence of non-granitic materials and the influence of man in the immediate surroundings. The bioindicator species most sensitive to this complex of parameters appear to be three species found only on churches (Caloplaca citrina, Sarcogyne davits and Verrucaria macrostoma) and the most widespread of the species found only on dolmens (Lasallia pustulata). Other important factors, probably affecting the influence of class of structure, are pH and the availability of nitrogen and moisture.