A Galerkin edge-element solution technique for Maxwell's equations in time domain is discussed. With proper diagonal lumping of one of the mass matrices, it can be seen as a generalization to a tetrahedral mesh and its barycentric dual of the staggered-grid finite difference scheme known nowadays as FDTD, or Yee's scheme. A new approach to the lumping, backed by a specific convergence-proof technique, is proposed. Copyright © 1999 John Wiley & Sons, Ltd.