In a previous paper (Bond and Scott, I955) evidence obtained from isotopic tests was given to show that the two lichens Collema granosum (Scop.) Schaer. and Leptogiumz lichenoides (L.) Zahlbr. fixed atmospheric nitrogen under experimental conditions. The circumstances indicated that at least the greater part of the fixation should be attributed to the Nostoc present in both these lichens. The possible presence of Azotobacter as a contaminant or symbiont was not denied, but it was argued that it was not present in sufficient quantity to account for any important part of the observed fixation. In the present paper a third lichen containing Nostoc as a symbiont, namely Peltigera praetextata (Flk.) Nyl., has been examined for evidence of fixation of nitrogen, firstly by the isotopic method and secondly by a study of the ability of disks of the lichen thallus to grow in the absence of combined nitrogen. Also, for comparison, isotopic tests for fixation have been carried out on a 'green' lichen, namely Cladonia impexa Harm. f. laxiuscula (Del.) Sandst. Frequent reference has been made to the existence of nitrogen-fixing bacteria in lichens since the postulation of the theory of poly-symbiosis by Cengia Sambo (I923) whose work concerned Azotobacter in the cephalodia of Peltigera aphthosa. These cephalodia were regarded by her and later by Goebel (I926) as being analogous to the root nodules of the legumes. The analogies drawn were, however, based on different suppositions. Cengia Sambo thought of the cephalodia as tubercles containing the nitrogen-fixing bacterium Azotobacter despite the strong disapproval of her compatriot Mameli Calvino (1925). Goebel, on the other hand, regarded the ATostoc in the cephalodia as being the nitrogen-fixer. It was not until some years later that Henckel and Yuzhakova (I936) demonstrated the presence of Azotobacter in epiphytic lichens. It is, however, to these authors that credit is generally, though erroneously, given for the introduction of the theory of poly-symbiosis. Henckel (I938) and Iskina (I938) extended these findings to include other groups of lichens and in particular the genus Cladonia from which Henckel and Yuzhakova had failed to isolate the bacterium. More recently Dr. G. Metcalfe (I955, personal communication) has shown Azotobacter to be present in a large number of British lichens including Cladonia-impexa. The tendency has thus been to extend the poly-symbiosis theory to include all lichens, but it must be noted that Walenkamp (Quispel, I945) and Krasilnikov (I949) have failed to detect Azotobacter in any of the considerable number of lichens examined by them. MATERIAL AND METHODS Cladonia impexa w-as collected from a Polytrichum-Calluna community and placed in flower pots, together Nith Sphagnum from the collection area, in a greenhouse and kept