We prove Galois equivariance of ratios of Petersson inner products of holomorphic cuspforms on symplectic, unitary, or Hermitian orthogonal groups. As a consequence, we show that the ratios of Petersson norms of such cuspforms with the same Hecke eigenvalues are algebraic. We also show that spaces of such cuspforms of sufficiently high fixed weight and level are spanned by theta series. (C) 2015 Elsevier Inc. All rights reserved.