Ribulosediphosphate carboxylase can be extracted from Pseudevernia furfuracea, either by Triton X-100 or by pretreatment with liquid nitrogen. With respect to HCO 3- the Km is 33 mM. The enzyme is competitively inhibited by sulfite, as is the case with spinach chloroplasts. The K~ value (t5.5-18.5 mM sulfite) indicates that in lichens the enzyme is not more sensitive than that in spinach. Thus the extreme sensitivity of lichens to SO 2 is not based on low tolerance at the enzymatic level, but is due to a low degree of avoidance.