We study the structure of the algebraic eigenspace corresponding to the spectral radius of a nonnegative reducible linear operator T, having a compact iterate and defined on a Banach lattice E with order continuous norm. The Perron-Frobenius theory is generalized by showing that this algebraic eigenspace is spanned by a basis of eigenelements and generalized eigenelements possessing certain positivity features. A combinatorial characterization of both the Riesz index of the spectral radius and the dimension of the algebraic eigenspace is given. These results are made possible by a decomposition of T, in terms of certain closed ideals of E, in a form which directly generalizes the Frobenius normal form of a nonnegative reducible matrix/ I. Introduction. Let £ bea Banach lattice, and T, a positive reducible linear operator mapping E into itself and having a compact iterate. Suppose, in addition, that r(T), the spectral radius of T, is positive. The primary purpose of this research is to ascertain, under what conditions on the Banach lattice E, a decomposition of T is possible, which turns out to be a natural generalization of the Frobenius normal form for reducible matrices. These properties will be seen to generalize those deduced by U. Rothblum [13] for the matrix setting, and those by H. D. Victory, Jr. for integral operators on LPspaces, 1 < p < oo, with the underlying measure being σ-finite [16, 17] on a domain set Ω.We refer the reader to the treatise by H. H. Schaefer [15] for an explanation of the notation used in this work and of the lattice concepts of ideals, order convergence and completeness, bands, projection bands, operator-invariant ideals, and (uniform) mean ergodicity of an operator T. By £(E), we mean the Banach space of bounded linear endomorphisms of E.The underlying Banach lattice E will be assumed equipped with an order continuous norm. In such Banach lattices, every filter that order converges norm converges. Such lattices are characterized by the fact that every closed ideal is a band [15, Theorem 5.14 (Chapter II)]. Since