Toll-like receptor 7 (TLR7) gene was identified and characterised from grass carp Ctenopharyngodon idella (designated as CiTLR7), and the mRNA expression profiles were examined in vivo and in vitro. The CiTLR7 genomic sequence consists of 4276 nucleotides (nts), including two exons and one intron. The full length of CiTLR7 cDNA sequence is 3354 nts with the longest open reading frame (ORF) of 3156 nts encoding a peptide of 1051 amino acids. CiTLR7 mRNA was high expression in spleen, skin and heart, and low expression in hepatopancreas, muscle, head kidney and trunk kidney in healthy fish. The expression of CiTLR7 was rapidly and significantly up-regulated at 6 h after grass carp reovirus (GCRV) injection (72.91 fold, P < 0.05), and recovered to the original level at 24 h post-injection in spleen. The CiTLR7 transcript was rapidly and significantly down-regulated at 6 h time point (0.32 fold, P < 0.05) and retrieved the normal level at 72 h post-injection in hepatopancreas. The CiTLR7 transcripts were rapidly and significantly inhibited at 2 h post GCRV infection in CIK (C. idella kidney) cell line (0.62 fold, P < 0.05), and were rapidly and significantly elevated by the stimulation of the synthetic double-stranded RNA polyriboinosinic polyribocytidylic acid sodium salt (poly(I:C)) in CIK in a dose- and time-dependent manners (P < 0.05). The results imply that CiTLR7 involves in the responses to double stranded RNA and virus infection.