ESR measurements confirm the existence of a nanocrystalline phase as well as an amorphous phase in boron nitride films grown by plasma-assisted chemical vapor deposition. The three observed spectra consist of a four-line component and a ten-line component superimposed on a broad single line. These hyperfine structures are assigned to a one-boron and a three-boron defect center located in the nanocrystalline regions whereas the single-line is associated with dangling bonds in the amorphous regions. Changes in the three spectra resulting from uv-irradiation and isothermal annealing will be reported.