During the last larval instar, dipteran insects synthesize two hexamerins rich in aromatic residues, typified by the larval serum proteins 1 and 2 (LSP-1 and LSP-2) of Drosophila melanogaster. We report here the characterization of a complete cDNA sequence encoding a LSP-1-like protein from a lower dipteran insect, the malaria mosquito Anopheles gambiae. The cDNA encodes the subunit of a homohexamer, A. gambiae hexamerin-1.1 (AgHex-1.1), which is a major pupal protein but only a minor constituent of late larval hemolymph. AgHex-1.1 is moderately rich in methionine (3.9%) and particularly rich in aromatic residues (21% Phe+Tyr). Cytogenetic analysis reveals AgHex-1.1 to be encoded by a single-copy gene localized to division 22F within the proximal 2La inversion breakpoint of chromosome 2 of A. gambiae. The AgHex-1.1 transcript is first detected in fourth-instar larvae (L4) and disappears abruptly in early pupae. In situ hybridization shows accumulation of the transcript uniquely in the larval fat body. AgHex-1.1 mRNA is re-expressed in male and female adults at about 10% of the L4 level, with no effect of bloodfeeding in females. The potential roles of AgHex-1.1 in Anopheles development and reproductive maturation are discussed.