A general expression is derived for the electrophoretic mobility of a soft particle, i.e., a spherical hard colloidal particle of radius a coated with a layer of polyelectrolytes of thickness d in an electrolyte solution. In the limit of d --> 0, the mobility expression tends to that for a spherical hard particle (Ohshima et al., J. Chem. Soc. Faraday Trans. 2 79, 1613 (1983)), whereas in the limit of a --> infinity , it tends to that for a plate-like soft particle derived previously (J. Colloid Interface Sci. 130, 281 (1989)). Also in the limit of a --> 0 and low potentials, the obtained mobility expression tends to a mobility formula for a spherical polyelectrolyte (a charged porous sphere with no particle core) derived by Hermans and Fujita (K. Ned. Akad. Wet. Proc. Ser. B 58, 182 (1955)). A simple approximate analytic mobility expression is presented. (C) 1994 Academic Press, Inc.