Abstract: To study the biological function of Smad1/5 in the growth and development of molluscs, the full-length cDNA of Smad1/5 in Meretrix meretrix (Mm-Smad1/5) was cloned via SMART RACE techniques. The entire cDNA of Mm-Smad1/5 was 1832 bp long, containing a 1380-bp open reading flame (ORF) that encoded a 459-amino acid protein. According to the amino acid sequence alignment, Mm-Smad1/5 shared 83.7% identity with Smad5 of C. gigas, 80.2% with Smad1 of P. fucata, and more than 70.5% identity with vertebrates. Protein functional domain predictions revealed that Mm-Smad1/5 contained both of the highly conserved functional domains MH1 and MH2. Together, these data indicate that Smad1/5 is highly conserved across evolution. The qRT-PCR results showed that Mm-Smad1/5 were expressed in all six tissues of adult clams and were expressed significantly more in the foot and mantle than other tissues (P<0.05). Furthermore, theMm-Smad1/5 gene was widely expressed in 10 developmental stages, it was first highly expressed in the gastrulae until the eyebot larva, largely decreased in the eyebot larva, and slightly increased in juvenile clams. Additionally, we identified 9 potential SNPs in the exons of Mm-Smad1/5 by DNA sequencing, of which the 936 G>T locus was significantly with the growth traits ofM. meretrix (P<0.05). The study indicated thatMm-Smad1/5 gene could be used as the growth candidate function gene, and its SNPs provided useful information for the reliable molecular markers for M. meretrix breeding.