The passive uptake of ammonium ions to extracellular cation binding sites has been demonstrated for a range of lichen species. The possible competitive effect of other cations on ammonium binding is discussed. Binding of uncharged gaseous ammonia to exchange sites was also shown. The rapid loss of exchangeable ammonium from extracellular sites has been demonstrated and, because the greatest losses occur in living material, intracellular uptake is inferred. The labile nature of bound ammonium means that transfer of material to the laboratory for analysis may result in the underestimation of the amount bound.