The phyllosemiquinone radical of the photosystem I reaction center has been studied by electron spin echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of ESEEM data of the semiquinone in N- and N- labeled PSI and numerical simulations demonstrate the existence of two protein nitrogen nuclei coupled to the semiquinone. One of the 14N couplings is characterized by a quadrupolar coupling constant eqQ/4h of 0.77 MHz, an asymmetry parameter η of 0.18, and a hyperfine coupling tensor with an almost pure isotropic hyperfine coupling, i.e. (A(xx), A(yy), A(zz)) = (1.3, 1.3, 1.5 MHz). The second nitrogen coupling is characterized by a quadrupolar coupling constant eqQ/4h of 0.45 MHz, an asymmetry parameter η of 0.85, and a weak hyperfine coupling tensor with a dominant anisotropic part, i.e. (A(xx), A(yy),A(zz)) = (-0.2, -0.2, 1.5 MHz). On the basis of a comparison of the N-ESEEM data with N-NQR and N-ESEEM data from the literature, the first coupled nitrogen is assigned to the indole nitrogen of a tryptophan residue. The coupling of the second nitrogen is much weaker and therefore more difficult to assign. However, the simulated spectrum best describes an amino nitrogen of a histidine, although the amide group of an asparagine or glutamine cannot be ruled out. The possible origins of the nitrogen hyperfine coupling are discussed in terms of the amino acid residues thought to be close to the semiquinone in PSI.