We examined dpp expression patterns in the pulmonate snail Lymnaea stagnalis and analyzed the functions of dpp using the Dpp signal inhibitor dorsomorphin in order to understand developmental mechanisms and evolution of shell formation in gastropods. The dpp gene is expressed in the right half of the circular area around the shell gland at the trochophore stage and at the right-hand side of the mantle at the veliger stage in the dextral snails. Two types of shell malformations were observed when the Dpp signals were inhibited by dorsomorphin. When the embryos were treated with dorsomorphin at the 2-cell and blastula stages before the shell gland is formed, the juvenile shells grew imperfectly and were not mineralized. On the other hand, when treated at the trochophore and veliger stage after the shell gland formation, juvenile shells grew to show a cone-like form rather than a normal coiled form. These results indicated that dpp plays important roles in the formation and coiling of the shell in this gastropod species.