The microbial growth and activity of acid phosphatase enzyme during the growth of Rhizopus delemar in the presence or absence of Ni(II) ions were investigated. An increase in initial Ni(II) ion concentration inhibited both growth rate of R. delemar and acid phosphatase activity. The maximum-intrinsic specific growth rate (mu m) and Monod constant (K(s)) of microorganism in Ni(II)-free medium were found as 0.0649 h(-1) and 1.8928 g/L, respectively. The inhibition of Ni(II) ions on growth rate of R. delemar was found to be a competitive inhibition and the inhibition constant was found to be 67.11 mg Ni(II)/L. The intrinsic Michaelis-Menten constant (K(m)) and maximum forward velocity of the reaction (v(m)) were determined as 3.17 mM and 833.3 mu mol/L min, respectively, in Ni(II)-free medium. In the presence of Ni(II) ions, the activity of acid phosphatase was inhibited. Addition of Ni(II) ions decreased the maximum reaction velocity, v(m), showed noncompetitive-type inhibition kinetics and the inhibition constant was determined as 50 mg Ni(II)/L. Maximum Ni(II) uptake was obtained by the growing cells of R. delemar, while the uptake capacity of resting cells was lowest. This study proved that acid phosphatase enzyme participated in the Ni(II) bioaccumulation mechanism of growing R. delemar. (C) 2010 Elsevier B.V. All rights reserved.