Urea acts as a nutritional inducer of urease in the lichen Evernia prunastri. Both photobiont and mycobiont seem to be able to produce the enzyme. However, the algal partner synthesizes urease during the first hours of culture while the fungal component collaborates later to maintain an adequate level of enzyme activity. Some of the urease activity separates as cell wall-pelletable protein whereas algal cells in axenic culture secrete the major fraction of the enzyme. The possibility of urease translocation from the photobiont to the mycobiont is discussed.