A novel protein in the hemolymph of newly collected mated red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), queens was identified using SDS gel electrophoresis and silver staining. The protein was observed 24 h postmating and was not present in the hemolymph of either virgin queens or in queens 2–14 d postmating. Using a reverse genetics approach, we isolated a 252-bp DNA fragment corresponding to this protein that has homology with the cysteine protease inhibitor (CPI) in plants. We suggest that this putative fire ant queen CPI is involved in flight muscle histolysis. The expression of the putative CPI was monitored at the transcriptional level to establish its role in flight muscle histolysis. A significant increase in the expression of putative CPI transcript was observed in mated queens compared with alate virgin queens. Enhancement in the expression of the putative CPI started from day 0 postmating and continued to increase for another 3 d. On day 7, the expression almost vanished. We discuss the possibility that the CPI protein inhibits proteasomal degradation in the flight muscle cells, rendering them subject to apoptosis.