The fungus Aspergillus niger produced extracellular and cellular acid phosphatase activities during growth in the presence or absences of copper ions in the medium. The levels of both phosphatase activities depended on the copper concentrations in the medium. Enzyme activities were maximal at the mid-exponential growth phase, as well as at the higher copper concentration and decreased as growth time increased from 12 to 48 h. The total uptake of copper (II) by mycelia, growing at the presence of copper was highest when the levels of enzyme activities were maximal, Between 10 and 20% copper (II) was not removed by washing in 0.1 M H2SO4, suggesting that this copper (II) was bound intracellularly by mycelia of different age. On the other hand, copper ions slightly inhibited cellular acid phosphatase activity of Aspergillus niger with a K-i of 0.89 mM. Acid phosphatase overproduction, as well as copper uptake of mycelia indicated a possible participation of acid phosphatases in the resistance of the fungal strain to the copper toxicity. (C) 2002 Elsevier Science Ltd. All rights reserved.