Structure of the haemoglobin from Limnodrilus gotoi (molecular weight, 3.01 x 10(6)) was studied electron microscopically and chemically. The molecule appears to be a regular hexagonal cylinder having the dimensions 220 A x 160 A, and to consist of 12 identical submultiples arranged six and six into two hexagonal discs laid one above the other. The protein contains 108 haems per molecule and 12 seryl, 12 threonyl, 35 glutamyl and 45 valyl residues to give a total number of 104 residues per molecule as N-terminal amino acids. These results lead us to a conclusion that the minimum molecular weight either per haem or per N-terminal residue is approximately 28,000 and the whole molecule consists of about 108 subunits or peptide chains, each containing single haem. Further it is presumed that submultiple is a nonamer, composed of nine subunits.