Infrared absorption bands for15N in crystalline silicon have been observed for the first time. Isotopic substitution experiments were performed by N+ion implantation into crystalline silicon to confirm the role of nitrogen in absorption bands at 764 and 962 cm−1for14N and at 748 and 937 cm−1for15N. Laser annealing increases the intensity of the higher frequency band. The 962‐cm−1band frequency is close to Si‐N stretch frequencies for planar bonded N in crystalline Si3N4. It is suggested that the lower frequency band is associated with a mode that is made IR active by local distortion, consistent with a distorted substitutional N center identified by Brower in electron paramagnetic resonance studies on laser‐annealed nitrogen‐implanted Si.