The Cu2+-tolerant yeast Yarrowia lipolytica accumulated Cu2+ until the late logarithmic phase. Thereafter, Cu2+ was temperature-dependently extruded into phosphate-limited culture medium containing high concentrations of heavy metal ions but not into 10 mM 2-(N-morpholino)ethane sulfonic acid (MES) buffer (pH 6.0). Peptone in the culture medium played an important role in the extrusion, which proceeded even when peptone was substituted with cysteine or histidine, but not with any other amino acid tested.