The cytology of the relationship between evolved lecanoralean lichen mycobionts and their trebouxioid photobionts were studied in the foliose Cetrelia olivetorum (Nyl.)Culb. & C. Culb., Platismatia glauca (L.)Culb. & C. Culb., and Parmelia tiliacea (Hoffm.)Ach., and in the fruticose Pseudevernia furfuracea (L.)Zopf (all Parmeliaceae) using SEM and partly TEM techniques. In freeze-dried SEM preparations of all four species crystalline secondary metabolites of fungal origin were detected on the surface of the algal symbiont. Extracellular lichen products crystallized within and on the outermost surface layer of the cell wall which spreads from the mycobiont over the surface of the photobiont. This outermost wall layer revealed an irregularly tessellated pattern in freeze-fracture preparations and was recognized as a very thin, electron-dense layer in ultrathin sections. According to histochemical tests the surface layer of medullary hyphae and algal cells is largely composed of proteins and polyesters of fatty acids. Published data on the in vitro effects of phenolic lichen products on the metabolism of isolated, cultured photobionts are summarized, and possible roles of these secondary metabolites at the myobiont-photobiont interface within the lichen thallus are discussed.