We report here the first examination of hexamerins expressed during mosquito larval development. Haemolymph proteins from fourth-instar larvae of six species representing the two major subfamilies of mosquitoes were characterized by immunoblotting using antisera to calliphorin, the major hexamerin of the blowfly,Calliphora vicina, or to LSP1 or LSP2, the two distinct hexamerins ofDrosophila melanogaster. In each mosquito species the antisera demonstrated the presence of multiple abundant hexamerin polypeptides of 66–85 kDa in molecular weight. According to the subunit composition of native proteins, the larval hexamerins from bothAedes aegyptiandAnopheles gambiaeform heterohexamers. Furthermore, the two majorAedeshexamerin subunits (AaHex1 and AaHex2) are neither rich in aromatic amino acids nor methionine. cDNA clones encoding AaHex1 and AaHex2 were isolated and used to show that hexamerin mRNA is uniquely expressed in fourth-instar larvae of bothA. aegyptiandA. gambiaeand disappears rapidly at the onset of pupal development.