The relationship between size of lichen thallus and net photosynthetic rate, respiration rate, rate of water uptake, rate of water loss and maximum thallus water content has been examined in five species of Umbilicaria. While two species show large effects of thallus size, three others show small, or no effects. The lack of a consistent pattern complicates the studies of physiological ecology in these and other lichen species because the vast majority of the published literature contains no mention of the dimensions of thallus or clones used. A major research effort should be made to determine other physiological functions that are age- or size-dependent. In addition, future work should include, where possible, information regarding dimensions of the thallus used and the range of dimensions normally found in the field. Only in this way can conclusions be tested by other workers.