The problem of the thermophoretic motion of a spherical particle in a gaseous medium situated at an arbitrary position between two infinite parallel plane walls is studied theoretically in the quasisteady limit of negligible Peclet and Reynolds numbers. The imposed temperature gradient is uniform and perpendicular to the plane walls. The Knudsen number is assumed to be small so that the fluid flow is described by a continuum model with a temperature jump, a thermal slip, and a frictional slip at the particle surface. The presence of the confining walls causes two basic effects on the particle velocity: first, the local temperature gradient on the particle surface is altered by the walls, thereby speeding up or slowing down the particle; second, the walls enhance the viscous retardation of the moving particle. A boundary collocation method is used to solve the thermal and hydrodynamic governing equations of the system. Numerical results for the thermophoretic velocity of the particle relative to that under identical conditions in an unbounded gaseous medium are presented for various cases. The collocation results agree well with the available approximate analytical solutions obtained by using a method of reflections. The presence of the walls can reduce or enhance the particle velocity, depending upon the relative thermal conductivity and surface properties of the particle as well as the relative particle-wall separation distances. The boundary effect on thermophoresis of a particle normal to two plane walls is found to be quite significant and generally stronger than that parallel to the confining walls. (c) 2006 American Institute of Chemical Engineers.