New and precise data on the 1 3 8 Ce 1 4 2 Ce ratio in marine manganese nodules obtained from the Atlantic and Pacific Oceans fall within a narrow range of -0.6 to +1.1 in terms of ε C e (one tenth of per mil difference from the standard). The nodule Ce isotopic data are used as a surrogate of surrounding seawater, and the ε C e values ranging from -0.6 to -0.2 for the central Pacific are clearly more negative than those (+0.2 to +1.1) for the Atlantic. By coupling the ε C e with ε N d for 1 4 3 Nd 1 4 4 Nd , it is inferred that the relative contribution of the continental-derived to mantle-derived sources is smaller in the Pacific than in the Atlantic. If we assume that the oceanic 1 3 8 Ce 1 4 2 Ce and 1 4 3 Nd 1 4 4 Nd ratios are determined by mixing of two end-members, i.e. the continental crust and MORB (mid-ocean ridge basalt), the oceanic variation of ε C e is greater than that of ε N d relative to their difference in the two end-members. This strongly suggests that Ce and Nd are fractionated within the ocean because of the difference in their mean residence times (90-165 yr for Ce and 1000-1500 yr for Nd) and the 1 3 8 Ce 1 4 2 Ce and 1 4 3 Nd 1 4 4 Nd ratios modified by scavenging and oceanic mixing are recorded in the manganese nodules.