(ReceivedWe report ferromagnetism at over 900 K in Cr-GaN and Cr-AlN thin films. The magnetic properties vary as a function of Cr concentration with 60%, and 20%, of the Cr being magnetically active at 3% doping in GaN, and 7% in AlN, respectively. In the GaN sample with the highest magnetically active Cr (60%), channeling Rutherford Backscattering indicates that over 70% of Cr impurities are located on substitutional sites. These results give indisputable evidence that substitutional Cr defects are involved in the magnetic behavior. While Cr-AlN is highly resistive, Cr-GaN exhibits properties characteristic of hopping conduction including T 1/2 resistivity dependence and small Hall mobility (0.06 cm 2 /V . s). A large negative magnetoresistance is attributed to the influence of the magnetic field on the quantum interference between the many paths linking two hopping sites. The results strongly suggest that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands. Cr-GaN, 5 and Cr-AlN. 6 (Ga,Mn)N films grown by molecular beam epitaxy were reported to be ferromagnetic above room temperature, with a Curie temperature, T c , of 940 K. 7 Another study of the same system synthesized using solid-state diffusion reported a T c in the range of 220-370 K, 3 while bulk Cr-doped GaN fabricated using the sodium flux method was reported to have a T c of 280 K. 5 The search for the physical mechanism responsible for the observed ferromagnetic properties and the question of the applicability of the classical magnetic models have become topics of intense interest. This letter reports the observation of ferromagnetism in Cr-GaN and Cr-AlN above 900 K, and describes the structural, electrical and magnetic properties of the materials.The Cr-GaN and Cr-AlN films were grown on 6H-SiC (0001) and sapphire (0001) substrates in a reactive molecular beam epitaxy system. Structural properties were characterized using X-ray diffraction ( Magnetic fields were applied parallel to the film plane during susceptibility measurements and perpendicular to the film during magnetoresistance (MR) measurements. The diamagnetic background contributions originating from the substrate and the sample holder were subtracted