Aspergillus niger, an acid-producing filamentous fungus, was immobilized on polyurethane foam. Various amounts of foam cubes and spore suspension were tested in order to obtain an efficient immobilization process. The best combination selected for further experiments was 0.2 g polyurethane foam and 3 ml spore suspension. Immobilized cells were reused, with higher levels of acid formation being maintained for longer periods (at least 240 h) than for free cells. The highest titratable acidity of about 315 mmole/l, was reached with 0.5 cm foam cubes after 3 x 48 h batch cultures in a laboratory shake-flask experiment. Rock phosphate (2.5 g/l and 5.0 g/l) solubilization was carried out in repeated 48 h batch fermentation using A. niger immobilized on polyurethane foam cubes. The greatest accumulation of soluble P, approximately 360 mu g/ml, was obtained after the second batch in flasks with 5 g/l rock phosphate, but process efficiency was higher at lower concentration of the rock phosphate. After 10 days of culture a total production of 1.2-1.6 mg/ml soluble P was obtained depending on the concentration of rock phosphate in the medium. (C) 1997 Elsevier Science Ltd.