Abstract. This paper gives a theory of spectral approximation for closed operators in Banach spaces. The perturbation theory developed in this paper is applied to the study of a finite element procedure for approximating the spectral properties of a differential system modeling a fluid in a rotating basin.Introduction. In this paper, we give a theory of spectral approximation for closed operators in Banach spaces. We then apply this theory to an analysis of the approximation of the spectral properties of some differential systems by finite element methods.Bramble and Osborn [1] and Osborn [14] developed a theory of spectral approximation for compact operators in Banach spaces. Their theory can be applied to the analysis of many numerical procedures for the spectral approximation of differential operators, T, such that T + XI has a compact inverse for some X G C. Most of the differential systems in the theory of elasticity are in this class.However, there are many differential systems of interest in mathematical physics which do not have compact resolvents. These operators can have continuous spectrum, eigenvalues of infinite multiplicity, and finite limit points of eigenvalues. Also, the eigenfunctions need not be smooth since the differential systems are not necessarily elliptic.Descloux, Nassif, and Rappaz [4], [5] have studied the approximation of the