Juvenile-hormone-dependent synthesis of vitellogenin in the fat body of migratory locusts is being studied for insight into subcellular modes of action of juvenile hormone (JH). Vitellogenin (Vg), a dimeric glycolipoprotein assembled from polypeptide subunits of 185 kDa, is produced in females from day 6–8 of adult life. Locusts deprived of corpora allata produce no Vg, but synthesis can be induced by administration of JH or an active analog such as methoprene. Vg synthesis could not be induced in male adults, but did occur in male larvae given high doses of methoprene, so that sex-limited cell programming must occur at metamorphosis. Adult fat body of both sexes responds to JH by DNA replication and enhanced protein synthesis. In stimulated female fat body, Vg mRNA of 6300 nucleotides is abundant. From locust genomic DNA libraries, clones making up two genes, VgA and VgB, have been isolated and shown to be the locust's only genes for Vg. Both are on the X chromosome and both contain introns. No cross-hybridization between them could be obtained for most of the coding regions, but similarity in their N-terminal amino acid sequences and those of chicken, Xenopus, and nematode Vgs suggests that these all belong to a gene superfamily. Repetitive sequences, including an Alu-like family, occur near the locust Vg genes. Assays with unique-sequence probes showed that, after a lag of about 24 and 12 h for primary and secondary stimulation, respectively, VgA and VgB mRNAs accumulate coordinately. Within 500 base pairs (bp) upstream from the transcription start sites, the sequences flanking VgA and VgB show several blocks of similarity; two of these, which are matched by 12-bp units upstream from JH-regulated cockroach oothecin genes, may represent JH-response elements. Locust Vg sequences transferred into Drosophila were integrated but not expressed. The data are consistent with a steroid-like model for control of gene transcription by JH, but many particulars remain to be established.