Hemolymph of a hemipteran insect, Riptortus clavatus, contains four distinct hexameric proteins (cyanoproteins) composed of two distinct subunits, CP alpha and CP beta, which show profiles of abundance depending on developmental stage, diapause status controlled by juvenile hormone, and sex. We have isolated cDNA clones encoding the two subunits and determined the complete sequences. The nucleotide sequences predict polypeptides of 693 and 691 residues for CP alpha and CP beta, respectively, including identical 16-residue signal peptides. The deduced amino acid sequences of both CP alpha and CP beta have significant similarity to other hemocyanin-related proteins, indicating that the cyanoproteins represent hexamerins. Phylogenetic analyses show that the cyanoprotein subunits can be grouped together with other hexamerins from exopterygote insects. Developmental Northern-blot analyses suggest that the expression of the cyanoproteins is regulated at the level of transcript abundance. In addition, the expression of the CP alpha subunit in female adults has been shown to be enhanced by juvenile hormone (JH) while the expression of the CP beta subunit is suppressed by the same hormone. To our knowledge, the CP alpha subunit of R. clavatus is the first case of a JH-enhanceable hexamerin whose sequence has been determined.