The amino-acid sequences of both subunits of phycoerythrocyanin from the thermophilic cyanobacterium Mastigocladus laminosus have been determined. The alpha-subunit consists of 162 amino-acid residues and has a molecular mass of 18200 Da. The beta-subunit is 171 residues long and has a molecular mass of 19600 Da. The tetrapyrrole chromophores are bound at position 84 in the alpha- and beta-subunits and at position 155 in the beta-subunit. The homology between the two subunits is 21%. The homologies between the phycoerythrocyanin subunits and the corresponding subunits of C-phycocyanin and allophycocyanin are 63% and 26% for the alpha-subunits and 67% and 36% for the beta-subunits, respectively. Secondary structure predictions were calculated for all six subunits of the phycobiliproteins from M. laminosus. The most conservative regions of the biliproteins were found in segments C-terminal to the chromophore-binding sites.