Fibroin gene expression during the larval developmental stages of the Saturniid silkworm, Antheraea mylitta, was analyzed. Northern blot analysis of larval silk gland total RNA using the fibroin gene as a probe showed that fibroin is expressed in the intermoult stages and repressed during the moulting stages. Abundance of fibroin transcripts gradually increased from the third to fifth intermoult stage, reaching a peak in the fifth intermoult. Transcripts declined during the early spinning stage. Western blot analysis of fibroin protein production with anti-fibroin antibody confirmed the differential fibroin expression, in accordance with fibroin mRNA synthesis. Dot blot hybridization of genomic DNA isolated from each larval developmental stage with the labelled fibroin gene showed that at the genomic level, the relative concentration of the fibroin gene was constant throughout the developmental stages. Our data confirm that fibroin gene expression in A. mylitta, like in B. mori, is transcriptionally controlled and shows differential temporal variations.