HSP60 is a highly immunogenic molecule, which is able to activate a large number ofT cell types and is implicated in a variety of autoimmune diseases.The grass carp (Ctenopharyngodon idella), a freshwater fish species of the family Cyprinidae, accounts for the third biggest value (USD 4.8 billion) at single species level of major cultured fish species in the world.Here, we isolated and characterized the HSP60 cDNA from grass carp (designated as CiHSP60).Its cDNA was 2434 bp in length and encoded a putative protein of 575 amino acids.BLAST analysis revealed that the CiHSP60 gene shared a high similarity with other known HSP60 sequences.CiHSP60 contained all three classical HSP60 family signatures.The mRNA of CiHSP60 was constitutively expressed in all tested tissues of untreated grass carp, including brain, muscle, trunk kidney, liver, head kidney, skin, spleen, heart, gill, intestine,and fin, with the highest expression level in the blood.CiHSP60 transcript was present in unfertilized eggs, which suggests that CiHSP60 transcription is maternally inherited.Fluorescent real-time quantitative RT-PCR was used to examine the expression of the CiHSP60 gene in grass carp after the challenge with the bacterium Aeromonas hydrophila.A clear time-dependent expression pattern of CiHSP60 was found after the bacterial challenge, and the mRNA expression reached a maximum level at three days post challenge, and returned to control levels after seven days.The upregulated mRNA expression of CiHSP60 in grass carp after bacterial challenge indicates that the HSP60 gene is inducible and involved in immune responses.These results suggest that CiHSP60 plays an important role in A.hydrophila-related diseases and in early embryonic development stages in grass carp.