Co immobilization by two manganese oxidizing isolates from Carlsberg Ridge waters (CR35 and CR48) was compared with that of Mn at same molar concentrations. At a lower concentration of 10 μM, CR35 and CR48 immobilized 22 and 23 fM Co cell(-1) respectively, which was 1.4 to 2 times higher than that of Mn oxidation, while at 10 mM the immobilization was 15-69 times lower than that of Mn. Scanning electron microscope and energy dispersive X-ray analyses of intact bacterial cells grown in 1 mM Co revealed Co peaks showing extracellular binding of the metal. However, it was evident from transmission electron microscope analyses that most of the sequestered Co was bound intracellularly along the cell membrane in both the isolates. Change in morphology was one of the strategies bacteria adopted to counter metal stress. The cells grew larger and thus maintained a lower than normal surface area-volume ratio on exposure to Co to reduce the number of binding sites. An unbalanced growth with increasing Co additions was observed in the isolates. Cells attained a length of 10-18 μm at 10 mM Co which was 11-15 times the original cell length. Extensive cell rupture indicated that Co was harmful at this concentration. It is apparent that biological and optimal requirement of Mn is more than Co. Thus, these differences in the immobilization of the two metals could be driven by the differences in the requirement, cell physiology and the affinities of the isolates for the concentrations of the metals tested.