Let beta be a finitely generated birational extension of Z[x], the ring of polynomials in one variable over the integers Z. (That is, B is a finitely generated extension of Z[x] contained in its quotient field Q(x).) Then Spec(B) is order-isomorphic to Spec (Z[x]). This affirms part of a conjecture of Wiegand (1986). (C) 1998 Elsevier Science B.V. All rights reserved.