An estimation of the dominant stresses in powders with or without interstitial gas flow yields a semiqualitative criteria for the transition between flow regimes (plastic-fluidized, plasticinertial, plastic-suspension, inertial-fluidized, inertial-suspension). It will be shown that, at ambient pressure, fine powders (particle diameter less than ~ 100 µm) experience a direct transition from plastic to fuidized regime, or from plastic to suspension. The inertial regime is absent in these fine powders. These ideas will be applied to understand the onset of fluidization in rotating drums. Previous measurements with xerographic toners (particle diameter around 10 µm) in rotating drums indicated that fluidized and plastic regimes coexist in the drum. The extent of the fluidized region grows when angular speed is increased. Here, we present measurements on the width of the horizontal surface of the fluidized zone as a function of rotation velocity, drum diameter and toner properties. We find that the results depend on the product of three fundamental parameters: (i) the ratio of the fluidized toner bulk density to the packed toner bulk density; (ii) the ratio of centrifugal acceleration to the acceleration of gravity and (iii) the ratio of gravitational potential energy per unit volume (or equivalently, kinetic energy per unit volume) to the powder tensile strength.