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Growth of Paracoccus denitrificans bacteria and Candida utilis yeast cells in the presence of increasing concentrations of cadmium led to an inhibition of cell multiplication. P. denitrificans was more resistant to the effect of cadmium than C. utilis. The pattern of cytochrome biosynthesis showed that cadmium inhibited cytochromes b and c synthesis in both organisms: In contrast while cadmium totally inhibited the biosynthesis of cytochrome c oxidase in C. utilis, it had a stimulatory effect on the biosynthesis of this enzyme in P. denitrificans. The comparison of colony size in cadmium–treated cells and in the control showed a remarkable decrease in the colony size of cadmium treated cells. The effect of increasing cadmium concentrations on colony count in C. utilis showed that from 0.006 to 0.125 mM cadmium the number of colonies was slightly but consistently higher than the control. At 0.25 mM cadmium the number of colonies decreased dramatically, reaching zero for 0.5 mM cadmium. This is the first reported evidence for an inhibitory (in eukaryote) and stimulatory effect (in prokaryote) of cadmium on the biosynthesis of cytochrome c oxidase. Moreover, our observations provide a better understanding of metal-microorganism interactions for bioremediation of metal contamination of the environment.