In this work, we study Joachimsthal's theorems in semi-Euclidean spaces E-nu(n+1). We obtain a relation between the curvatures of the strips in semi-Euclidean spaces E-nu(n+1) in matrix form depending on a semi-orthogonal matrix. This matrix equation gives us a generalization of the original Joachimsthal theorem in semi-Euclidean space E-1(3) and the well-known Joachimsthal theorem of the surface theory. In addition, we reobtain the well-known Joachimsthal theorem from our matrix equation in the case n = 2, nu = 0 and the original Joachimsthal theorem from our matrix equation in the case n = 2, nu = 1. (C) 2008 Elsevier Ltd. All rights reserved.