A mixed finite element Galerkin method is analyzed for a strongly damped wave equation. Optimal error estimates in L-2-norm for the velocity and stress art: derived using usual energy argument, while those for displacement are based on the nonstandard energy formulation of Baker. Both a semi-discrete scheme and a second-order implicit-time discretization method are discussed, and it is shown that the results are valid for all t > 0. (C) 2001 John Wiley & Sons. Inc.