We propose an efficient numerical method for a non-selfadjoint Steklov eigenvalue problem. The Lagrange finite element is used for discretization. The convergence is proved using the spectral perturbation theory for compact operators. The non-sefadjointness of the problem leads to non-Hermitian matrix eigenvalue problem. Due to the existence of complex eigenvalues and lack of a priori spectral information, we propose a modified version of the recently developed spectral indicator method to compute (complex) eigenvalues in a given region on the complex plane. In particular, to reduce computational cost, the problem is transformed into a much smaller matrix eigenvalue problem involving the unknowns only on the boundary of the domain. Numerical examples are presented to validate the effectiveness of the proposed method.1. Introduction. Steklov eigenvalue problems arise in mathematical physics with spectral parameters in the boundary conditions [24]. Applications of Steklov eigenvalues include surface waves, mechanical oscillators immersed in a viscous fluid, the vibration modes of a structure in contact with an incompressible fluid, etc [24,13,14]. Recently, Steklov eigenvalues have been used in the inverse scattering theory to reconstruct the index of refraction of an inhomogeneous media [12]. Note that most Steklov eigenvalue problems considered in the literature are related to partial differential equations of second order. However, Steklov eigenvalue problems of higher order were also studied, e.g., the fourth order Steklov eigenvalue problem [2].In contrast to the theoretical study of the Steklov eigenvalue problem, numerical methods, in particular, finite element methods have attracted some researchers rather recently [3,6,7,25,14,1,23,16,21]. Various methods have been proposed, including the isoparametric finite element method [3], the virtual element method [25], non-conforming finite element methods [16,21], the spectral-Galerkin method [2], adaptive methods [6], multilevel methods [32], etc. All of the above works consider the selfadjoint cases. In this paper, we consider a non-selfadjoint Steklov eigenvalue problem arising in the study of non-homogeneous absorbing medium in inverse scattering theory [12]. There seems to exist only one paper by Bramble and Osborn [9], which considered the non-selfadjoint case. However, the second order non-selfadjoint operator is assumed to be uniformly elliptic and no numerical results were reported in [9]. In this sense, the current paper is the first paper contains both finite element theory and numerical examples for a non-selfadjoint Steklov eigenvalue problem, to the authors' knowledge. For the general theory and examples of finite element methods for eigenvalue problems, we refer the readers to the book chapter by Babuška and Osborn [4], the review paper by Boffi [8], and the recently published book by Sun and Zhou [31].There are two major challenges to develop effective finite element methods for non-selfadjoint eigenvalue problems [26,4,31]. The first one is the l...