Hexamerins are proteins found in high abundance in the haemolymph of larval and adult insects. The expression patterns of the genes encoding the house fly, Musca domestica, hexamerins were determined by Northern analyses using cDNAs as probes. A cDNA, A1, hybridized to a fat body-specific messenger RNA (mRNA) which is detectable in larvae until pupation. Antibodies raised to the larval-specific hexamerin, Hex-L, bind recombinant protein encoded by a 5' rapid amplification of cDNA ends (RACE) product of A1, A2, indicating that the A cDNAs likely represent the genes encoding Hex-L. The F1, F2 and F3 cDNAs, corresponding to genes encoding an adult, female-enriched hexamerin, Hex-F, hybridized with an mRNA isolated from protein-fed females which has a temporal expression profile similar to that observed for the accumulation of Hex-F. Furthermore, expression of the mRNAs hybridizing to the F cDNAs is correlated with the abundance of Hex-F protein during the gonotrophic cycles. The mRNA transcription profiles indicate that the Hex-L and Hex-F genes are regulated in a sex-, tissue- and developmental phase-dependent manner. This stage-specific expression of hexamerins contrasts with the expression patterns of hexamerins seen in other insects. The conceptual translation products of larval hexamerin cDNAs showed identity with larval serum protein 1 (LSP1)-type hexamerins while the deduced products of the female hexamerin cDNAs showed the highest identity with LSP2-type hexamerins. Genomic analyses showed that the larval hexamerin and female hexamerin genes from M. domestica belong to two distinct multigenic families.