The structure of tantalum oxynitride has been determined by the profile analysis of powder neutrondiffraction data collected at 4.2°K. The compound is monoclinic, space group P21/c, with a=4-9581, b=5"0267, c= 5"1752 A., and fl= 99.640 ° (4.2°K). Complete ordering of the anions occurs in alternate layers normal to the [100] direction and this ordering is consistent with calculated site potentials. Short nitrogen-nitrogen distances of 2.51 to 2"64 A~ are observed.