Major histocompatibility complex class II antigens are important in vertebrate immune system.In the present study,the full cDNA sequence of class II A gene was synthesized by RACE-PCR from half-smooth tongue sole(Cynoglossus semilaevis),and its open reading frame(ORF) polymorphism was studied.The whole cDNA sequence was 992 bp in length,including the ORF with 717 bp.Twenty-five alleles were identified and clustered into two distinct groups according to the specific nucleotides/amino acids in specific positions.Eleven alleles belonged to Cyse-DAA while fourteen alleles belonged to Cyse-DBA.Four Cyse-DAA alleles were observed in one individual,and three to five Cyse-DBA alleles were observed in each of the three detected individuals,which indicated that at least two loci existed in each gene.Moreover,in order to study the function of the alleles in resistance to infection,200 individuals were intraperitoneally injected with Vibrio anguillarum and the first 20 dead individuals and 20 surviving ones were selected for genotype analysis.Fifty-six alleles were identified among the 40 individuals.Twenty-nine alleles belonged to Cyse-DAA and the other 27 alleles belonged to Cyse-DBA.Eighteen alleles were selected for studying their function in resistance to infection.Alleles Cyse-DAA*0201,Cyse-DAA*1101,Cyse-DBA*0401,Cyse-DBA*1102,Cyse-DBA*1801 and Cyse-DBA*2201 were identi-fied only in surviving individuals,while alleles Cyse-DAA*0901,Cyse-DBA*1101 and Cyse-DBA*1401 occurred more frequently in dead individuals.This study confirmed the existence and polymorphism of two class II A genes as well as the relationship between alleles of class II A genes and disease susceptibility/resistance in half-smooth tongue sole. Major histocompatibility complex class II antigens are important in vertebrate immune system. In the present study, the full cDNA sequence of class II A gene was synthesized by RACE-PCR from half-smooth tongue sole (Cynoglossus semilaevis), and its open reading frame ( ORF) polymorphism was studied. The whole cDNA sequence was 992 bp in length, including the ORF with 717 bp. Twenty-five alleles were identified and clustered into two distinct groups according to the specific nucleotides / amino acids in specific positions. Eleven alleles belonged to Cyse-DAA while fourteen alleles belonged to Cyse-DBA. Four Cyse-DAA alleles were observed in one individual, and three to five Cyse-DBA alleles were observed in each of the three detected individuals, which indicated that at least two loci existed in each gene. More than, in order to study the function of the alleles in resistance to infection, 200 individuals were intraperitoneally injected with Vibrio anguillarum and the first 20 dead individuals and 20 survivors iving ones were selected for genotype analysis. Ftyty-six alleles were identified among the 40 individuals. Twenty-nine alleles belonged to Cyse-DAA and the other 27 alleles belonged to Cyse-DBA. Eleighteen alleles were selected for studying their function in resistance to infection.Alleles Cyse-DAA * 0201, Cyse-DAA * 1101, Cyse-DBA * 0401, Cyse- DBA * 1102, Cyse- DBA * 1801 and Cyse- DBA * 2201 were identi- fied only in surviving individuals, while alleles Cyse -DAA * 0901, Cyse-DBA * 1101 and Cyse-DBA * 1401 occurred more frequently in dead individuals.This study confirmed the existence and polymorphism of two class II A genes as well as the relationship between alleles of class II A genes and disease susceptibility / resistance in half-smooth tongue sole.