The weathering phenomena resulting from the growth of six foliose and crustose lichens (Parmelia subrudecta, Xanthoria ectaneoides, Parmelia conspersa, Aspicilia radiosa, Caloplaca sp. and Ochrolechia parella) on three mafic rocks have been studied. The bioweathering results in more or less extensive fragmentation and corrosion of the mineral surfaces immediately beneath the lichen thalli and in the formation, in the thallus or at the rock-lichen interface, of secondary products. The significant amounts of whewellite found in all interfaces, and the bipiramids of weddellite detected at the serpentine rock-Ochrolechia parella interface, suggest that the oxalic acid secreted by the mycobiont is the chemical substance principally involved. The capacity of the lichens to alter their rock substrata does not appear to be related to their thallus morphology.