10.22376/ijpbs.2019.10.1.p1-12 Volume 3 Issue 1 2012 (January - March) Fishes inhabit the microbe-rich aquatic environment, hence are constantly under the threat of being infected with the microbial pathogens. Non-specific immune system of fish forms the first line of defense in protecting them from the invading pathogens. Toll-like receptors (TLRs) are important components of non-specific immunity that play an essential role in the recognition of the pathogens based on pathogen associated molecular patterns (PAMPs) and in inducing the non-specific immunity. TLR9 recognizes the unmethylated CpG DNA of bacteria and viruses. As the expression level of TLRs in a host is an indicator of its health status and its ability to defend the pathogen challenge, the aim of this study was to compare the tissue-specific expression profiles of TLR9 mRNA in lessThan i greaterThan Danio rerio lessThan /i greaterThan (zebra fish) that were healthy and experimentally exposed to lessThan i greaterThan Aeromonas hydrophila, lessThan /i greaterThan a gram-negative bacterium as it has not been reported so far. Observations of the study shows that tissue-specific differential expression of TLR9 mRNA is present in skin, gill, brain, liver, intestine and kidney of zebra fish and it could be up-regulated upon exposure (4 h) to lessThan i greaterThan Aeromonas hydrophila lessThan /i greaterThan . A.Uma, G.Rebecca And K.Saravanabava Aermonas hydrophila infection, mRNA expression, TLR9, Zebra fish. 520-526