Experimental results are reported which confirm the validity of the fluid dynamic scaling rules for gas-solid fluidised beds operated in the bubbling regime, regardless of the particle and wall. roughness. These rules still apply for shallow beds operated in the slugging regime for cases in which the ratio (H-mf/D-t) of inital bed height to bed diameter is less than the tangent of the angle of internal friction of the particles; however, for higher values of H-mf/D-t, which permit the formation of fully developed dense-phase slugs, the scaling rules become invalid. This is considered to be due to particle-particle particle-wall interactions, and evidence is reported for both these effects in the slugging regime.