Recent reports have revealed that hemocyanin in shrimp and other invertebrates is a novel multifunctionalprotein with immunological activities, such as phenoloxidase, antiviral, antibacterial, hemolytic, and antitumor activities.Moreover, several studies—including our own previous findings—indicate that hemocyanin may produce some functionalfragments involved in pathogen resistance in shrimp. However, no study has comprehensively evaluated thesefragments. We conducted a comparative analysis in the proteome of the shrimp Litopenaeus vannamei after the shrimpwere infected with Vibrio parahaemolyticus for 24 h to search for novel hemocyanin fragments. Tricine-sodium dodecylsulfate-polyacrylamide gel electrophoresis and Western blotting analyses of hemolymph drawn directly from the pericardialsinus revealed five peptide fragments (bands a, c, e, f, g), ranging from 6.0 to 31.0 kD, that bound to anti-shrimphemocyanin antibodies in plasma, compared with those in the control group, suggesting that they might be hemocyanindegradation products. Moreover, a 6.0 kD peptide fragment (named HMCp6) was subjected to matrix-assisted laserdesorption/ionization-tandem time-of-flight mass spectrometry analysis, and the database search results indicated thatthe HMCp6 peptide fragment was highly homologous with two subunits of L. vannamei hemocyanin (gi|325462537 andgi|854403). This result is similar to the finding that the C- and N-terminal hemocyanin fragments are significantlyaltered in Penaeus vannamei hemocytes during Taura syndrome virus infection. The HMCp6 peptide fragmentpossessed clear antibacterial activities (antibacterial rate, 98%) against V. parahaemolyticus. This result agrees with aprevious finding that a 1.9 kD antibacterial peptide from Pacifastacus leniusculus hemocyanin possesses clear antibacterialactivities against Gram-positive bacteria. In conclusion, our data suggest that the HMCp6 peptide fragment mightbe a novel fragment derived from hemocyanin with a pathogen-resistance function. These results will assist in investigationsof the mechanism of these hemocyanin fragments and help to establish strategies for controlling diseases inshrimp.