Diastereomers of isoleucine have been investigated on the insoluble fractions of 41 molluscan fossils yielded from the Pleistocene deposits in the Kanto district. The results are as follows. 1) The rations of D-alloisoleucine to L-isoleucine (D/L) show different values even in the fossils belonging to the same stratigraphic horizon, probably owing to the recent amino acid addition to the fossil ones in the shells. 2) The maximum D/L ratio in each formation which represents the value with the least contamination of recent L-isoleucine, becomes greater with an increasing in geological ages of the Pleistocene formations treated in this paper. Racemization of isoleucine could be a useful tool for the stratigraphic correlation among the Pleistocene formations. 3) The reaction rate constant of L-isoleucine to D-alloisoleucine in the fossils of the Narita Formation is 5.0×10^-7 yr.^-1 from the calculation based on the following values in eq. (2) ; D/L=0.07 and absolute age by the fission track method=1.4x 10⁵yr. B. P. 4) The absolute age of the Yabu Formation is calculated at 2.6×10⁵yr. B. P. by using this rate constant to eq. (2), which is in good agreement with the age 2.75×10⁵yr. B. P. determined by the fission track method.