Crustin is a cysteine-rich antibacterial peptide which widely distributes within decapod crustaceans. In the present study, the cDNA encoding crustin-like peptide (designated CrusEs) was cloned from Chinese mitten crab Eriocheir sinensis by using rapid amplification of cDNA ends (RACE) approaches and expressed sequence tag analysis. The full-length cDNA of CrusEs was of 796bp, containing a 5' untranslated region (UTR) of 214bp, a 3' UTR of 267bp with a poly(A) tail, and an open reading frame (ORF) of 315bp encoding a polypeptide of 104 amino acids including a putative signal peptide of 21 amino acids. A WAP domain and the consensus framework existing in class I crustins were both identified in CrusEs, suggesting that CrusEs is a new member of type I crustins. Quantitative real-time RT-PCR was employed to examine the expression of CrusEs, and its mRNA transcript was mainly detected in haemocytes and gill. The CrusEs mRNA transcript in haemocytes was down-regulated after the challenge with Gram-positive bacteria Micrococcus luteus, while Gram-negative bacteria Listonella anguillarum did not induce significant variation of CrusEs mRNA. The mature peptide of CrusEs was cloned into pET-21a(+) with a C-terminal hexa-histidine tag fused in-frame, and expressed in Escherichia coli, and the recombinant CrusEs inhibited the growth of Gram-positive bacteria with low MIC. The results indicate that CrusEs is a potent antibacterial protein against Gram-positive bacteria infection, and it may play an important role in innate immune response of E. sinensis.