The mechanism of particle segregation in a jig bed has been investigated in order to achieve optimal utilization of this device. The motion of a tracer particle penetrating a jig bed consisting of equisized spheres was monitored as a function of jigging speed and stroke length. The measured particle velocities appear to be a function of stroke length and frequency. A special optical technique was used to trace an opaque particle in a bed of transparent particles immersed in a fluid medium of the same refractive index.