The cDNA of silkworm (Bombyx mori) antichymotrypsin (sw-Achy) was cloned from larval fat body and its nucleotide sequence was determined. The deduced amino acid sequence of mature sw-Achy begins with Phe1 and ends with Phe384, with a preceding 16-amino-acid signal peptide. The amino-acid sequence similarities of sw-Achy with the serine-proteinase inhibitors (serpins) silkworm antitrypsin, tobacco hornworm alaserpin, human alpha-1-antitrypsin and human alpha-1-antichymotrypsin were 29.6%, 30.3%, 26.1%, and 25.0%, respectively. The highly conserved amino acids in other serpins are also conserved in sw-Achy. sw-Achy is thought to be a new member of the serpin family. Multiple alignment of sw-Achy with 23 other kinds of serpin by the progressive method produced a phylogenetic tree in which all four insect serpins are grouped separately within one branch. The reactive site of sw-Achy with alpha-chymotrypsin was identified as Thr343-Ser344 by direct amino-acid sequence analysis of cleaved and purified protein.