Abstract. For α ∈ R, let Dα denote the scale of Hilbert spaces consisting of Dirichlet series f (s) =The Gordon-Hedenmalm Theorem on composition operators for H 2 = D 0 is extended to the Bergman case α > 0. These composition operators are generated by functions of the form Φ(s) = c 0 s+ϕ(s), where c 0 is a nonnegative integer and ϕ(s) is a Dirichlet series with certain convergence and mapping properties. For the operators with c 0 = 0 a new phenomenon is discovered: If 0 < α < 1, the space Dα is mapped by the composition operator into a smaller space in the same scale. When α > 1, the space Dα is mapped into a larger space in the same scale. Moreover, a partial description of the composition operators on the Dirichlet-Bergman spaces A p for 1 ≤ p < ∞ are obtained, in addition to new partial results for composition operators on the DirichletHardy spaces H p when p is an odd integer.