The influence of a clinoptilolite-rich tuff from Bigadiç area, Turkey on the concentration of biomass of P-accumulating bacteria and the phosphate (P) uptake was investigated. Experiments were carried out in reactors containing the experimental wastewater with different initial concentrations of P inoculated with the pure culture of Acinetobacter calcoaceticus. The addition of a clinoptilolite-rich tuff in the aerobic phase of experiment, when compared with control reactors, resulted in the significantly higher biomass of free cells and the higher amount of P removed from wastewater. The P-uptake rates per single cell were lower in reactors with clinoptilolite-rich tuff addition, which is explained by the higher concentration of cells that are actively multiplying. The main contribution of the clinoptilolite-rich tuff to the P removal from wastewater was the increase of biomass, and in a lesser extent the adsorption of P on the zeolitized tuff. © 2005 Elsevier B.V. All rights reserved.