JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The Johns Hopkins University Press is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Mathematics.In this paper we establish some basic arithmetic results concerning automorphic forms in several variables. Our results are, in part, analogues of the classical facts for elliptic modular forms, and results of Shimura on Hilbert and Siegel modular forms, [9] and [10]. In essence, this involves relating rationality properties of Fourier-Jacobi coefficients to rationality properties in algebraic geometry.Our automorphic forms are C-valued holomorphic functions on certain Cartesian powers Hnd of Siegel upper half-spaces Hn. We consider appropriate arithmetic groups r acting on Hnd such that r\H,d is noncompact. In particular, we reduce our study to that of congruence subgroups r of reductive linear algebraic groups G such that the derived group G' is almost simple over Q. Further, let K be a maximal compact subgroup of G'(R)+, the connected component of the identity in the real points of G'. We only consider G such that G'(R)+/K is isomorphic to Had. Up to an irrelevant central torus, all such groups G are obtained as totally indefinite quaternion unitary groups (see [8] and [15]). Therefore, we are able to describe very explicitly the action of such groups on the appropriate domains.It will become clear that the groups obtained as unitary groups over even-dimensional quaternionic vectorspaces, or from quaternion matrix algebras, have a simple nature: for such groups, one can write Fourier expansions for automorphic forms in the same way that one does for Hilbert of Siegel modular forms. In particular, the Fourier coefficients are numbers.The groups arising from odd-dimensional vectorspaces over a quater-nion division algebra are more complicated: we can only write a Fourier-Jacobi expansion as in [6]. Specifically, we use coordinates zz uX (z, u, T) = Vtu TJ on H d +1 such that z is in H2nd, T is in H1d, u is in C2nd. An automorphic form f will have a Fourier-Jacobi expansion f(z, u, T) = i ax(u, T) exp(trxz), x as x ranges over a suitable lattice of d-tuples of positive semidefinite 2n-by-2n real matrices. Here ax is a theta function in the u variable, and expL(T)ax(uo, T) is an automorphic form on H1d, when uo is a division point of the period lattice of ax, and L is a holomorphic function of T, which depends on uO. For these groups and domains (among others), Shimura describes in [8] a field of functions with special arithmetic properties. Their values at CM-points in the domain generate some abelian extensions of related CMfields. By the results of [2], the quotients r\H d have normal irreducible quasi-projective models. By [8], there exist m...