Previous studies suggested that resistance to Bacillus thuringiensis (Bt) endotoxins Cry1Ab and Cry1Ac in a strain of the European corn borer (ECB) was not associated with differences in toxin binding. The activity of soluble trypsin-like proteinases in the resistant ECB larvae was significantly reduced and was associated with decreased activation of Cry1Ab protoxin. The objective of this study was to understand the molecular basis for reduced proteinase activity in resistant ECB larvae. The full-length cDNA sequences of three putative trypsin genes (Pm2, Pm25, and Pm23) were determined in Bt-susceptible and -resistant ECB larvae.  Pm2 contained a 1571-base pair (bp) nucleotide sequence with a 1386-bp open reading frame (ORF) encoding a 462-amino acid protein.  Pm25 consisted of a 937-bp sequence with a 768-bp ORF encoding 256-amino acid protein.  Pm23 contained a 975-bp sequence with a 771-bp ORF encoding a 257-amino acid protein. A number of differences were found in the comparison of nucleotide sequences of Pm23 in resistant and susceptible ECB larvae.  One of these differences was a proline residue at position 190 in Pm23 from the susceptible strain, changed to a histidine residue at the same location in the sequence from the resistant strain.  This difference can result in structural and/or charge differences in the resistant trypsin-like proteinase, and because it is near the conserved substrate-binding region, may affect the activity of the proteinase.  Further studies on the correlation of the alterations to proteinase activities and mRNA expression levels of the three genes are underway.