Model of water transport in a membrane electrode assembly of a polymer electrolyte fuel cell (PEFC) is developed. The model takes into account nonlinear diffusion of liquid water in the membrane. It is shown that nonlinearity leads to the formation of closely located dry and wet regions in the membrane. The model is incorporated into our quasi-3D model of a PEFC and recent experiments of Buchi and Scherer are simulated. The model qualitatively reproduces measured membrane resistance and the mean profile of water content across the membrane. Calculated two-dimensional distribution of water in the cell cross section is discussed. (C) 2003 The Electrochemical Society.