An Atlantic herring major histocompatibility class II A (Clha-DAA) cDNA sequence has been characterized and was shown to encode a leader peptide, alpha-1 domain, alpha-2 domain, connecting peptide, transmembrane and cytoplasmic region. The Clha-DAA protein sequence has all the characteristics of a teleost class II A protein with conserved cysteines in both the alpha-1 and the alpha-2 domains and two potential N-linked glycosylation sites. Exon 2 sequences encoding the polymorphic alpha-1 domain from different individuals were analysed and revealed the presence of at least two loci. The Clha-DAA gene consists of four exons and three short introns. Four unique intron 3 sequences from multiple individuals were obtained and were shown to contain a (TG)n microsatellite sequence. Primers were optimized such that only a single microsatellite locus designated Clha-DAA-INTR3 was amplified. Four herring populations from the North Sea and the Baltic Sea were genotyped for Clha-DAA-INTR3. In total, 16 Clha-DAA-INTR3 alleles were detected; the distribution of the alleles showed no deviation from Hardy–Weinberg expectation. Levels of genetic differentiation among samples were of similar magnitude as have been reported earlier for neutral microsatellite loci between northern North Sea and Baltic Sea herring populations.