Microvitellogenin and vitellogenin cDNA from Manduca sexta (tobacco hornworm) were tested for use as molecular probes to investigate the expression of genes coding for vitellogenins in Spodoptera frugiperda (fall armyworm) and Lymantria dispar (gypsy moth). Cross-hybridization was not observed between the M. sexta cDNAs and S. frugiperda DNA and mRNA. Vitellogenin cDNA from M. sexta did not hybridize to L. dispar DNA or mRNA. However, the 834 bp microvitellogenin cDNA from M. sexta hybridized to an approximately 850 bp transcript in L. dispar mRNA. A 2.5 kb cDNA clone, pz64, was isolated from late last instar larvae of female L. dispar by differential screening. This clone has 38% amino acid sequence (deduced) and 55% nucleic acid sequence similarities with the 3'-end of high molecular weight vitellogenin in Bombyx mori (silkworm). When used as a probe in northern analysis of L. dispar mRNA, this cDNA hybridized to a 5.3 kb transcript in female last instar larvae, pupae, and adults, but not to male last instar larvae and adults. This cDNA did not hybridize to mRNA from M. sexta or S. frugiperda. Expression of the 5.3 kb vitellogenin transcript hybridizing to the 2.5 kb cDNA clone was suppressed in 5-day-old last instar larvae of female L. dispar treated on day 2 with doses of the juvenile hormone analog, methoprene, greater than 10 nmol. Apparently, the high in vivo titer of juvenile hormone during the first 2 days of the last instar represses the transcription of vitellogenin mRNA.