To understand the molecular mechanisms of the larval-specific transcription of Bombyx mori larval serum protein (BmLSP), we isolated a clone of the BmLSP gene from a genomic library and sequenced a 3.5-kb fragment. An intron was found in the 5' noncoding region of the BmLSP gene. A putative transcription start point was determined by primer extension analysis. Genomic Southern hybridization showed that there is one copy of the BmLSP gene in a haploid genome. A database search revealed that the BmLSP gene has presumptive repetitive sequences found in other B. mori genes, the sequence homologous to ecdysone-responsive elements and a heptamer sequence found in storage protein genes.