We present a method for determining the δ18O of seawater in the deep ocean during the last glacial maximum from the measured δ18O values of deep sea pore fluids. Using data from Deep Sea Drilling Project (DSDP) site 576 in the Western Pacific, this method yields a glacial to interglacial change in δ18Oswof 1.0±0.25‰. This value for Δδ18Oswis the first direct measurement of deep ocean δ18O for the last glacial maximum and avoids the problems of spatial and temporal variability of the δ18O of surface water implicit in previous determinations. More precise, higher‐resolution pore fluid measurements are required to improve this de