The ratio Sr 87/Sr 86 in samples of water and shells of the peleoypod Mytihis edulis, Linné, from the Hudson Bay was found to be 0.7093 ± 0.0003 which is identical to the value of this ratio in the northern Atlantic Ocean. The concentrations of strontium and calcium in four water samples from the eastern Hudson Bay vary linearly with salinity and are appreciably lower than in the open ocean. The ratio Sr × 10 3/ Ca in the Hudson Bay was found to be equal to 18.7 ± 0.3 and does not differ significantly from values obtained for surface water from the Atlantic Ocean. Concentrations of strontium and calcium in Lake Superior are uniform throughout and average Sr = 21.8 ± 0.4 ppb, Ca = 14.3 ± 0.5 ppm. The ratio Sr × 10 3/Ca is 1.53 ± 0.04. Concentrations of the same elements in Lake Huron exhibit regional variations and are significantly higher than in Lake Superior. The Sr × 10 3/Ca ratio is approximately 3.5. The distribution coefficient for Sr+ 2 for Mytilus edulis in the Hudson Bay ranges from 0.14 to 0.20 and is significantly lower than experimentally determined values in carbonates composed of a mixture of aragonite and calcite. The average distribution coefficient for Sr+ 2 in the aragonitic shells of Lamposilis is 0.256 ± 0.027 (σ). The concentration of strontium in shells of Lampsilis fluctuates widely, but appears to be controlled, to a first approximation, by the distribution coefficient of Sr+ 2 in calcium carbonate and water. Attention is drawn to the discrepancy between the Sr × 10 3/Ca ratios of surface run-off water and the modern ocean. The apparent enrichment of strontium over calcium in the oceans is an important aspect of the marine geochemistry of strontium and calcium.