Sperm whale myoglobin, an oxygen-storage hemoprotein, was reconstituted with 2,7-diethyl -3,6,12,17-tetramethyl-13,16-bis(carboxyethyl)porphycenatocobalt(II) in order to investigate the reactivities of a cobalt porphycene in a protein matrix. Similar to the previously reported finding for the myoglobin with the iron porphycene, the reconstituted myoglobin with the cobalt porphycene was also found to have an 02 affinity 2 orders of magnitude greater than that of the myoglobin possessing cobalt protoporphyrin X The EPR spectra of the deoxy and oxy myoglobins having the cobalt porphycene at 77 K also have features similar to those of the myoglobin with cobalt protoporphyrin IX. These spectra suggest that the porphycene cobalt in the deoxy form is coordinated by one nitrogenous ligand postulated to be the imidazole ring of His93, and that the bond configuration Of Co-II-O-2 is regarded as the (CoO2.-)-O-III species.