Free and encapsulated cells of Yarowia lipolytica produced citric acid in repeated batch-shake-flask fermentation on either calcium carbonate or rock phosphate supplemented medium, the latter being simultaneously solubilized by the organic acid. Agar-encapsulated cells on calcium carbonate supplemented medium produced the highest amount of citric acid productivity of 0.234 g/l per h. Alginate and k-carrageenan appeared to be unsuitable cell-carriers when rock phosphate was supplied to the medium solution instead of calcium carbonate. The soluble phosphorus concentration depended on the mode of biocatalyst application (as freely suspended or encapsulated cells) and the initial concentration of rock phosphate in the cultivation medium. Rock phosphate clearly affected the behaviour of Y. lipolytica and determined the level of acid productivity and corresponding P-solubilization. (C) 2000 Elsevier Science Ltd. All rights reserved.