Measurements have been made of axial solids mixing in fluidized beds of 0.05, 0.1, and 0.22 m diameter with a variety of materials at atmospheric pressure. Round nosed slugs, raining slugs, and turbulent bubbling have been investigated. Raining slugs occur with coarser materials. With cracking catalyst round nosed slugs form readily in the smaller diameter beds, but in the largest bed, breakdown to the turbulent state was normal. A model has been developed to predict the axial solids mixing in a slugging fluidized bed containing round nosed slugs. The model supposes that each slug is followed by a well-mixed wake and a piston flow region. The model predicts well the scale effect noted by W. G. May. Axial mixing is fast for round nosed slugs, falls for turbulent bubbling, and is low for raining slugs.