The potential for photorespiratory gas exchange has been examined in Nostoc commune which grows as surface colonies in prairie-grassland, as well as in its lichenized form, Collema furfuraceum. Closed-loop 14CO2-12CO2 methods were used together with an examination of CO2 compensation point and the effects of low O2 tension. The results show unequivocally with all methods, that there is no photorespiratory activity in N. commune. Conversely in C. furfuraceum there is a marked inflection in the 14CO2 uptake rate after 3 min in the light; this is followed, on transfer of the lichen to darkness by an immediate and substantial 12CO2 and 14CO2 release which subsequently declines. The interpretation of the latter phenomenon as a classical post-illumination photorespiratory 14CO2 burst is negated by the total absence of any stimulation of light respiration (LR) at low oxygen tension. A more acceptable interpretation in the strong metabolic channelling of recent photosynthate from the phycobiont. We emphasize the need for several independent measures before the presence of photorespiration (PR) in lichens can be firmly established.