There was determined experimentally the residence time distribution of solids forming fluidized phase and moving in a, co-current flow with fluidizing gas throughout voids of bed of spheres randomly packed within a column. The column was of 0.1 m inside diameter; the spheres, 0.01 and 0.015 m in diameter, were packed to the height of 1 m. The fluidized phase was generated of sands by air stream at ambient conditions and with interstitial velocities corresponding to the fluidization numbers of 6-12 in individual experiments; the sands fraction was collected between the sieves with the meshes of 0.25.10(-3) and 0.315.10(-3) m. The results of experiments show that the residence time distribution of the solids forming the fluidized phase in the open system investigated is similar-to that characteristic for the flow tank system with ideal mixing in the flowing medium. No effects of the fluidization number as well as of the diameter of the spheres forming the bed have been observed.