Abstract: Cytochrome P450 (CYPs) enzymes are responsible for the metabolism of various kinds of endogenous and exogenous compounds in mammals. In fish, CYP3A plays an important role in the detoxification of xenobiotics. In the present study, a full-length sequence of CYP3A cDNA in silver carp was cloned and sequenced, and then a phylogenetic tree of CYP3A was constructed. Additionally, the acute toxicity of chlorpyrifos on silver carp and the effect of chlorpyrifos on CYP3A expression were also evaluated in this study. The results of bioinformatics prediction revealed that the full CYP3A encoded a protein of 513 amino acids and the molecular weight of CYP3A protein was 58.8 ku and the isoelectric point was 7.96. This protein was stable and hydrophilic, and it had 45.2% of a-spin, 12.3% of extended strand, 4.3% of β-fold, 38.2% of random coil and two binding-domain of crossing-membrane. Meanwhile, the results of acute toxicity tests indicated that chlorpyrifos was highly toxic to silver carp and the 96h LC50 of chlorpyrifos on silver carp was 0.172 mg/L. Finally, the results of Q RT-PCR indicated that chlorpyrifos significantly down regulated the expression of CYP3A at mRNA level in the liver of silver carp.