A simple technique based on microscopy and computer digitalized images is used to record the three-dimensional trajectory of colloidal particles, The vertical motion is controlled with the help of a piezoelectric actuator and a procedure has been developed in order to keep the particle in the focal plane. This technique allows one to obtain simultaneously the hydrodynamic radius and the density of a particle by following both the Brownian motion and the sedimentation. A demonstration of the capability of the method is given for micrometer-size latex particles. With semitransparent electrodes it is also possible to determine the electrophoretic mobility of a particle. We show that this determination is more accurate than using electrophoretic light scattering when the particles have a low mobility, We apply this technique to determine the diameter, the density, and the zeta potential of silica particles in silicone oil.