The possibility of using ammonia as a source of nitrogen for the molecular beam epitaxy (MBE) of AlGaAsN/GaAs layers was studied. It is shown that nitrogen is not incorporated into the GaAs layers in a broad range of the MBE conditions studied. The incorporation of nitrogen is possible in the presence of aluminum in the growing film. The molar fraction of nitrogen in the growing material is equal to that of aluminum, provided that the supply rate of ammonium is sufficiently high. Theoretical estimates are confirmed by the experimental data. (C) 2002 MAIK "Nauka/Interperiodica".