The wall effects on the terminal falling velocity of a sphere moving through power law liquids (1 greater than or equal to n greater than or equal to 0.36) contained in a rectangular duct have been examined in the creeping flow region. Defining an effective diameter D-E of the rectangular duct with a falling sphere, the experimental wall factor data have been correlated by means of a single relationship for rectangular ducts of arbitrary geometry including the parallel plates and square ducts.