Removal of the areolae of the crustose lichen Rhizocarpon geographicum (L.) DC. resulted in either low or no measurable radial growth of the marginal hypothallus. Radial growth of the hypothallus was also significantly reduced compared with intact thalli when (I) aerolae were removed to within 1 and 2 mm of the hypothallus and (2) a 5 mm wide ‘moat’ was created between the areolae and the hypothallus. Adding ribitol (0.01 M) to isolated hypothalli at 3-month intervals over 15 months resulted in total radial growth c. 60% that of intact thalli. Adding an arabitol/mannitol mixture (0.05 M arabitol, 0.03 M mannitol) increased radial growth compared with deionized water and ribitol treatments. Adding ribitol (0.7M), arabitol (0.2 M) and mannitol (0.08 M) to the areolae of intact thalli had no significant effects on radial growth of the hypothallus. On a south-facing rock surface, isolated hypothalli grew at a similar rate to intact thalli for 2 months. Growth then declined and the hypothalli disappeared from the rock surface within 6 months. The effects of addition of carbohydrate suggests that the marginal hypothallus has the capacity to utilize exogenous materials. However, in intact thalli in the field, the radial growth of the hypothallus is likely to be a result of transfer of materials from the areolae through hyphal connections.