Abstract: Ikaros is a kind of transcription factor with zinc finger structure that is essential to the development of lymphocyte. In order to reveal the structure characteristic of Ikaros gene and the immune modulatory mechanisms of Ikaros gene on disease resistant, the cDNA of Ikaros were obtained from Oreochromis niloticus with RT-PCR and RACE methods, and the genomic DNA of Ikaros were obtained by using PCR and Genome Walking technique in this study. Quantitative real-time PCR was used for analyzing the organization distribution and the response of Ikaros to Streptococcus agalactiae infection. The genomic DNA is 20545 bp with 7 introns and 8 exons encoding 6 kinds of mRNA splicing isoform via alternative splicing. The deduced amino acid sequences of the six splicing isoforms with zinc finger domain have high homology (70.6%—93.7%) in other teleost fish. The Ikaros gene expressed in all tested tissues, which strongly expressed in blood, and moderately expressed in thymus, spleen and head kidney. After challenged with pathogenic bacteria Streptococcus agalactiae, the Ikaros gene expression levels rose in blood, thymus, spleen and head kidney, and then reached to the peak after 48 hours. The results suggested that Ikaros gene participated in the immune response of Nile tilapia against Streptococcus agalactiae infection. This research laid a foundation for further study on the disease resistant mechanism of Ikaros gene.