The present contribution deals with the thermophoresis particle deposition effect on the free convection over a horizontal flat plate embedded in a fluid-saturated porous medium. In a certain sense, this study continues that by Chamka and Pop [A. Chamka, I. Pop, Effect of thermophoresis particle deposition in free convection boundary layer from a vertical flat plate embedded in a porous medium, Int. Commun. Heat Mass Transfer 31 (2004) 421-430], where the vertical flat plate configuration was analyzed. The governing equations are transformed into a set of coupled differential equations, which are solved numerically using a finite difference method. For various values of the problem parameters, graphs of the profile concentration in the boundary layer and of thermophoretic deposition velocity are presented. (C) 2007 Elsevier Ltd. All rights reserved.