In this study, reservoir water level effects on nonlinear dynamic response of arch dams are investigated. For this purpose, the nonlinear behaviour of the dam concrete is idealized as elasto-plastic using the Drucker-Prager model based on the associated flow rule assumption. Water in the reservoir is represented by the Lagrangian (displacement-based) fluid finite elements. The program NONSAP is modified for elasto-plastic analysis of fluid-structure systems and employed in the response calculations. Nonlinear dynamic analysis of an arch dam subjected to earthquake ground motion is performed for five different water levels. The El-Centro N-S component of the Imperial Valley earthquake, on May 18, 1940, has been used as the ground motion. The crest displacements, the maximum tensile stresses on the upstream and downstream faces of the dam and the time history of the yield function of an element in the dam body are presented. The results obtained from nonlinear analyses for different water levels are compared with each other. It is apparent that the reservoir water level effects must be considered in the elasto-plastic analyses of arch dams to earthquake ground motion. (c) 2007 Elsevier Ltd. All rights reserved.