Peltigera didactyla (With.) J. R. Laundon has a cosmopolitan distribution, but in the Antarctic it is very restricted and is nowhere common or abundant, occurring at a number of sites in the northern maritime Antarctic extending to 65°S (Øvstedal & Lewis-Smith, 2001). Within its Antarctic distribution it appears to be most frequent on Signy Island, South Orkney Islands. Typically, this species grows as small colonies amongst mosses in relatively dry, gravelly, sheltered habitats, forming aggregations of lobed sorediate thalli up to c. 10–15 (rarely to c. 25) cm diameter.