Fluorescence emission spectra at 4 K of the D1-D2-Cytb559 reaction center complex of photosystem II, presumably containing six chlorophyll alpha (Chl alpha) and two pheophytin alpha (Pheo alpha) molecules, were obtained by selective excitation with a tunable, continuous laser (lambda(exc) = 680-691 nm). The bands in the vibrational region (700-800 nm) showed a characteristic structure, which could be used as a "fingerprint" to identify the emitting pigment at low temperature. By comparison of this "fingerprint" with those of monomeric Chl alpha and Pheo alpha in Triton X-100 detergent, it is unambiguously show that the steady-state emission arises from a Chl alpha species for lambda(exc) > 683 nm. The nonselectively excited emission spectrum (lambda(exc) < 679 nm) reveals primarily Chl a emission bands, peaking at 683.7 +/- 0.3 and 744.0 +/- 0.5 nm. The shape of this spectrum is significantly different from that of earlier reported emission spectra of D1-D2-Cytb559 complexes with presumably a lower Chl alpha content (four to five Chl/2 Pheo). These show characteristic bands for Pheo alpha emission. We conclude that D1-D2-Cytb559 complexes with a higher Chl alpha content (6 Chl/2 Pheo) contain an additional long-wavelength (680-683 nm) Chl alpha molecule, which is not related to P-680. The steady-state fluorescence at low temperatures mainly arises from emission of this Chl and not from charge-recombination fluorescence of P-680.