To clarify the molecular mechanism of prevention of entry into diapause in Bombyx mori by HCl treatment, we biochemically analyzed mitochondrial Ca² -dependent solute carrier protein (MCSC) in diapause eggs treated with HCl solution. Our previous studies revealed that HCl treatment causes Ca² to efflux from diapause eggs. Therefore, we attempted to analyze MCSC, which is known to associate with Ca² . The isolated cDNA of B. mori MCSC (BmMCSC) had an open reading flame (ORF) of 667 amino acid residues, and the ORF contained two EF-hand calcium-binding domains and three characteristic features of the mitochondrial solute carrier superfamily. The gene expression level of BmMCSC increased by HCl treatment. A Ca² binding assay indicated that recombinant BmMCSC (rBmMCSC) shows an affinity with Ca² . The distribution of BmMCSC was investigated with an immunohistochemical technique using antisera against BmMCSC in diapause eggs and HCl-treated diapause eggs. BmMCSC was localized in serosa cells in both eggs. These data may suggest that BmMCSC is activated by intracellular Ca² or efflux Ca² by HCl treatment, and that it plays a role in the molecular mechanisms of artificial diapause prevention or the breaking of diapause in the silkworm.