Abstract. In this paper, an iterative solution method for a fourth-order accurate discretization of the Helmholtz equation is presented. The method is a generalization of that presented in [10], where multigrid was employed as a preconditioner for a Krylov subspace iterative method. This multigrid preconditioner is based on the solution of a second Helmholtz operator with a complexvalued shift. In particular, we compare preconditioners based on a point-wise Jacobi smoother with those using an ILU(0) smoother, we compare using the prolongation operator developed by de Zeeuw in [37] with interpolation operators based on algebraic multigrid principles, and we compare the performance of the Krylov subspace method Bi-CGSTAB with the recently introduced induced dimension reduction method, IDR(s). These three improvements are combined to yield an efficient solver for heterogeneous high-wavenumber problems.Key words. Helmholtz equation, non-constant high wavenumber, complex-valued multigrid preconditioner, algebraic multigrid, ILU smoother AMS subject classifications. 65N55, 65F10, 78A45, 35J051. Introduction. Many authors, e.g. [5,7,13,19], have contributed to the development of appropriate multigrid methods for the Helmholtz equation, but an efficient multigrid treatment of heterogeneous problems with high wavenumers arising in engineering settings has not yet been proposed in the literature. The multigrid method [4,14] is known to be a highly efficient iterative method, for example, for discrete Poisson-type equations, even with fourth-order accurate discretizations [6,33]. The Helmholtz equation, however, does not belong to the class of PDEs for which off-the-shelf multigrid methods perform efficiently. Convergence degradation and, consequently, loss of O(N ) complexity are caused by difficulties encountered in the smoothing and coarse-grid correction components; see [7,33] for a discussion.We present an efficient numerical solution technique for the heterogeneous highwavenumber Helmholtz equation, discretized by fourth-order finite differences. Recently,in [10], a robust preconditioned Bi-CGSTAB method has been proposed for solving these problems, in which the preconditioner is based on a second Helmholtz equation with an imaginary shift. This preconditioner is a member of the family of shifted Laplacian operators, introduced in [20], and its inverse can be efficiently approximated by means of a multigrid iteration. Two-dimensional results, representative for geophysical applications, generated by second-order finite differences, have been presented in [26] and 3D results in [27].In this paper, we generalize this solver and include, in particular, a fourth-order discretization of the Helmholtz operator in our discussion. The multigrid preconditioner is enhanced, in the sense that we replace the point-wise Jacobi smoother in the multigrid preconditioner by a variant of the incomplete lower-upper factorization smoother, ILU(0). Furthermore, we evaluate the performance of a prolongation