Cells of Candida utilis grown in continuous culture under the influence of rather high concentrations of copper ions have been studied biochemically, morphologically, and cytologically. Although under these conditions a new steady state has been adjusted, the population is much more differentiated than under control conditions. With regard to their size, structure, and viability the cells have been arranged into 4 groups. Generally, the "copper cells" in comparison with the "normal cells" are more voluminous (average: 2.5 times; maximum: 4 times) and of altered structure. The appearance of dense particles ("copper containing particles"), globules ("lipid globules"), vacuoles, and the thickening of the cell wall, as well as the disappearance of the mitochondrial cristae have been shown as characteristic indications for damage in the cell substructure caused by the influence of copper ions. Occasionally abnormal cell shapes can be observed. A permanent influence of copper ions in the concentration used was followed also by striking disturbances of the cell metabolism. The cells take up more carbon, phosphorus, and nitrogen, whereas protein synthesis and respiratory activity decrease. Both the synthesis of lipids and polysaccharides and the phosphohydrolase activity increase. A discussion of the experimental data is presented.