A cDNA for a midgut chymotrypsin, induced by a blood meal, has been cloned and sequenced from the mosquito Aedes aegypti. The 938 base sequence codes for a 268 amino acid protein, which contains an 18-residue signal peptide and a seven-residue activation peptide. The deduced amino acid sequence contains several features typical of chymotrypsin proteases, including the catalytic triad of serine proteases and the residues that determine the chymotrypsin substrate specificity pocket. The chymotrypsin mRNA, absent in larvae, pupae, males and newly emerged females, reaches detectable levels within 24 h post-emergence and attains a maximum level 3-7 days after emergence. Translation of the chymotrypsin mRNA is induced by feeding a protein meal, and there is a dramatic increase in midgut chymotrypsin enzymatic activity after feeding. Chymotrypsin activity remained high during protein digestion, but chymotrypsin protein levels and enzymatic activity were almost undetectable once digestion was completed, 48 h after feeding.