Biological soil crusts are important cryptogamic communities covering the sand dunes of the north-western Negev. The biological crusts contain cyanobacteria and other free-living N(2)-fixing bacteria and are hence able to fix atmospheric nitrogen (N). This is why they are considered to be one of the main N input pathways into the desert ecosystem. However, up to now, in situ determinations of the N(2) fixation in the field are not known to have been carried out. We examined the natural (15)N method to determine the biological N(2) fixation by these soil crusts under field conditions. This novel natural (15)N method uses the lichen Squamarina with symbiotic green algae--which are unable to fix N(2)--as a reference in order to determine N(2) fixation. Depending on the sampling location and year, the relative biological fixation of atmospheric nitrogen was estimated at 84-91% of the total N content of the biological soil crust. The cyanobacteria-containing soil lichen Collema had a fixation rate of about 88%. These fixation rates were used to derive an absolute atmospheric N input of 10-41 kg N ha(-1) year(-1). These values are reasonable results for the fixation of atmospheric N(2) by the biological crusts and cyanolichens and are in agreement with other comparable lab investigations. As far as we are aware, the results presented are the first to have been obtained from in situ field measurements, albeit only one location of the Negev with a small number of samples was investigated.