Let be a Siegel modular form of degree , weight and character for the congruence subgroup of the Siegel modular group . Suppose that is an eigenfunction for all Hecke operators with index relatively prime to . It is proven that for each fixed, symmetric, semi-integral, positive definite matrix of order and for each Dirichlet character , equal to zero on all prime divisors of , the Dirichlet series where are the Fourier coefficients of and is the set of integral matrices of order with positive determinant, has an expansion as an Euler product which can be explicitly calculated.Bibliography: 13 titles.