Proton nuclear magnetic resonance spectra are reported for cytochrome cd1 from Pseudomonas aeruginosa (ATCC 19429) in several forms including complexes of the ferricytochrome with cyanide, azide, and fluoride, a quasi-apo form in which the noncovalently associated heme d1 has been removed but the covalently bound heme c is retained, and the reduced state of both native and the quasi-apo forms. Comparisons are made to the previously reported spectrum of ferricytochrome cd1. The following points are made. The spectra of the azide and fluoride complexes and the ferric quasi-apo form show perturbation of resonances assignable to the site of heme d1, and leave relatively unperturbed resonances assignable to the site of heme c. The heme d1 associated resonances are at 46.0, 35.4, 23.3, 17.5, -2.9, and 16 ppm, and the heme c associated resonances are at 42.0, 33.7, 15.0, 13.9, -7.5, -14, and -33 ppm in native ferricytochrome cd1. The similarity of the hyperfine resonances of the ferric quasi-apo from to the heme c resonances of intact ferricytochrome cd1 is evidence that removal of heme d1 leaves the heme c binding site relatively unaltered. Linewidths and relaxation times suggest that the relaxation times of the unpaired electron spins of the ferric hemes c and d1 are on the same order of magnitude. Although it is paramagnetic, ferrocytochrome cd1 does not demonstrate an experimentally detectable hyperfine shifted spectrum under present conditions. Possible reasons for this are discussed. The presence of a narrow resonance at -2.8 ppm in both ferrocytochrome cd1 and the reduced state of the quasi-apo form suggests that methionine may be a ligand to heme c.