The well-known and widespread lichen species Cladonia furcata (Huds.) Schrad. is usually very constant in its chemistry: fumarprotocetraric acid is its main secondary metabolite, sometimes accompanied by atranorin. Recently a new chemical strain, characterised by the presence of psoromic acid instead of fumarprotocetraric acid or atranorin, was found in Portugal by the first two authors during phytosociological investigations of heath vegetations. The plants are preserved in the herbarium of the Institute of Systematic Botany, University of Utrecht (U), leg. Barendregt & v.d. Dries nr. 1-2 (U). Morphologically the plants with psoromic acid represent the slender form of C. furcata. which is the predominant form in lowland western Europe (fig. 1). The podetia are c. 3 cm long and up to 0.8 mm wide, branching regularly but not very densily dichotomously, and olivaceous green to brownish in colour. Their habit varies from creeping and loosely tufted to erect and densily tufted. Squamules are present only occasionally, on the lower parts of the podetia, and are roundish with a crenulated margin, up to c. 1.2 mm wide.