The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and plays an important regulatory role in immune response. The AhR is central to the functions of T cells, monocytes and dendritic cells (DCs) and the production of inflammatory cytokines. To understand the roles of grass carp (Ctenopharyngodon idella) ahr1a gene, we cloned the cDNA of the ahr1a gene in grass carp. The ahr1a cDNA of C. idella is 2 893 bp in length, encoding a protein of 847 aa. It was predicted that grass carp AhR1a protein contains a typical domain of the bHLH-PAS (Period-Aryl hydrocarbon receptor nuclear translocator Single Minded, Pre-Arnt-Sim) subfamily which belongs to the helix-loop-helix (HLX) superfamily. Sequence analysis showed that the amino acid similarity and identity between grass carp AhR1a and common carp (Cyprinus carpio) were the highest. Phylogenetic tree analysis showed that the AhR1a of C. idella was clustered in a clade with homologous amino acid of goldfish (Carassius auratus), common carp and zebrafish (Danio rerio), all of which belong to the cyprinid family. Quantitative real-time PCR (qRT-PCR) analysis found that ahr1a was constitutively expressed in multiple tissues including intestine, gills, kidney, liver, head kidney, thymus, spleen, skin and brain, with the highest expression detected in the intestine and gills. Infection with Flavobacterium columnare resulted in significant decreases of the ahr1a expression in the spleen (24,48 h) and head kidney (12,24 h). In contrast, infection of grass carp reovirus (Reovirus of grass carp, GCRV) did not alter the expression levels of ahr1a in the spleen and head kidney. In the primary head kidney leukocytes, lipopolysaccride (LPS) and recombinant IL-1β dowgregulated the expression of ahr1a. In conclusion, the grass carp ahr1a gene was analyzed in this study. The results suggest that the ahr1a of C. idella may be involved in regulating inflammatory response.