Experiments were carried out to estimate the frequency of collisions and the collisional particle pressure in an inverse three-phase fluidized bed. The collisional particle pressure is the additional pressure exerted on the wall of a system due to the impacts of the particles. The measures were taken by means of a high-frequency-response hydrophone. The effects of solid hold-up and gas velocity were investigated. The frequency of the collisions appeared to increase with the solid hold-up, up to a given value, and decrease for solid amounts approaching the fixed bed state. The particle pressure variation with solid hold-up was a bell-curve whose shape could be both represented by the mechanistic model of Batchelor (J. Fluid Mech. 193 (1988) 75) and the biofilm abrasion rate suggested by Gjaltema, van Loosdrecht and Heijnen (Biotechnol. Bioeng. 55(1) (1997) 206). A direct correlation for estimating the frequency of collisions and the particle pressure as a function of the operating parameters is proposed.