Jak kinase is a non-receptor type tyrosine kinase,which is broadly found in mammals,birds,insects and fish. It is an important factor in Janus kinase/signal transduction and transcription factor signaling pathway,which is widely involved in the processes of immunity,cell proliferation,growth,differentiation,migration and apoptosis. TYK2 is the first member of the Jak family to be identified and has been confirmed to play a key role in cytokine transduction. In this article,the ORF sequence of the TYK2 gene in Megalobrama amblycephala was cloned and analyzed. Results showed that the ORF sequence of the TYK2 gene in M. amblycephalawas 3 489 bp in length encoding 1 162 aa. The M. amblycephala TYK2 (MamTYK2) is consisted of 23 exons and 22 introns,similar to other vertebrate species. The MamTYK2 protein is composed of 4 domains including B41,SH2,TyrKc (pseudokinase domain) and TyrKc (tyrosine kinase domain). TYK2 is highly conserved through evolution. Based on the results of quantitative realtime PCR (qRT-PCR),among healthy adult tissues,TYK2 is highly expressed in the middle kidney,followed by the blood,spleen,and head kidney,and lowly expressed in other tissues including the intestine,brain,gill,heart,muscle,and liver. After infected by Aeromonas hydrophila,the TYK2 mRNA showed a similar expression pattern in the spleen and intestine:significantly decreased at 4 hpi (hour post infection) (P＜0.05) and reached a minimum,and significantly increased at 12 hpi (P＜0.05) and reached a maximum. It significantly increased and reached the maximum value at 24 hpi (P＜0.01),and significantly decreased and reached the minimum value at 36 hpi (P＜0.01) in the midkidney. It is indicated that the MamTYK2 gene may play an important role in the process of M. amblycephala resistance against A. hydrophila.