A model has been constructed to predict assimilation in four lichen species using climatological data. Gas exchange of the lichens has been measured in the laboratory in response to seasonal, light, temperature and moisture changes. By fitting polynomial equations to the response curves, the model predicts assimilation on the basis of these variables. A comparison of weight changes measured in the field with assimilation values predicted by the model using climatological data measured in the field, shows that the model gives realistic predictions over periods of 1 month. The model also permits the importance of the various environmental parameters to be examined, singly and in combination. Predicted assimilation values on climatically different days suggest that moisture is the single most important factor governing lichen growth – the assimilation gains during one rainy summer day can counteract the losses of five dry days. The model can now be extended to examine possible reasons for lichen distribution.