Abstract: intercellular adhesion molecule-1（ICAM-1） is an important adhesion molecule mediating cell recognition and adhesion, and it is involved in a series of important physiological and pathological processes including cell signal transduction and activation, inflammatory response and immune response. The full length of ICAM-1 cDNA of Nile tilapia（Oreochromis niloticus）（named On-ICAM-1） was cloned and identified in this study. The bioinformatics analysis of this gene was carried out, and the expression pattern of On-ICAM-1 m RNA in various tissues after the stimulation of Streptococcus agalactiae was studied by using fluorescence quantitative PCR method. The results showed that the ORF was 1 059 bp, encoded 352 amino acids, with a molecular weight of 38.8 kD and a theoretical pI of 5.55. The sequence of On-ICAM-1 was similar to that of Astatotilapia calliptera, Haplochromis burtoni and Pundamilia nyererei in 89.80%, 88.10% and 71.02%. Amino acid sequence of the protein of the gene alignment showed that ICAM-1 was highly conserved in other cichlid fishes. Moreover, the mRNA expression levels in different tissues were analyzed by real time quantitative PCR, it was found that the expression of On-ICAM-1 could be detected in all the 10 examined tissues and the highest expression level was in the thymus, with lower level detected in gill, intestine, skin, and the lowest level was detected in head kidney. Under Streptococcus agalactiae stimulation, the expression of On-ICAM-1 gene was significantly down-regulated in gill, intestine, spleen, brain and thymus, and the expression was temporal. The expression levels in gill and spleen were the lowest at 12 h, and decreased expressions in intestine, brain and thymus at 12 h, all of which were upregulated at 24 h. This study showed that On-ICAM-1 expression could be induced by Streptococcus agalactiae and was involved in the immune response of Nile tilapia.