Single‐crystal Si samples with nitrogen (N) impurities (Si:N) and with N and phosphorus (P) impurities (Si:N:P) have been investigated by electron spin resonance measurements. It was found that substitutional N impuritiesNscannot be incorporated into Si by cw laser annealing of N ion‐implanted Si or by N doping during crystal growth; however,Nsis incorporated into Si by pulsed‐laser annealing (PLA) of N ion‐implanted Si. The spin density ofNsdecreases with doping of P shallow donors into PLA Si:N and increases by introduction of slight point defects in PLA Si:N:P. These results suggest thatNswith a negative charge are formed in PLA Si: N:P system.