The normalized cDNA library from liver of Sinonovacula constricta was constructed using the SMART cDNA construction kit and large numbers of colonies were randomly picked and sequenced.Two EST sequences with high homology with β-actin gene of other species were found and then the complete express sequence of one from S.constricta was obtained by PCR and 5′RACE, named β-ACTIN 1.The cDNA of this gene was 1 552 bp,which consists of a 73 bp 5′ untranslated region(UTR),a 1 131 bp open reading frame(ORF)and a 348 bp 3′ UTR.The translated protein is composed of 376 amino acids,with 41.95 ku molecular weight,and its calculated isoelectric point was 5.23.Compared with the other 7 molluscs of amino acid sequences,the amino acid sequence of β-ACTIN 1 inS.constricta has twelve specific amino acid residues:Ile179,Glu229,Ser232,Pro236,Ile248,Asn272,Cys273,Val283,Ser320,Ser325,Val330,Pro339,respectively.Meanwhile,the amino acids sequence of β-ACTIN 1 in S.constricta shared the high similarity with the other 7 molluscs(more than 97%).Phylogenetic analysis suggested that S.constricta clustered with mollusca firstly,and then clustered with arthropoda,finally clustered with fish,amphibians,mammals.The quantitative reverse transcriptase(qRT-PCR)analyses showed that the expression level of β-ACTIN 1 gene was not stable in different tissues and after the Vibrio anguillarum induced in S.constricta. So this β-actin gene was not suitable as an internal control.