Oxygen isotope values of sequentially microsampled accretionary carbonate from the thick-shelled calcitic bivalve Eurydesma from the Early Permian of southeastern Australia vary seasonally over 6 yr of growth. Paleotemperature constraints derived from coeval glendonite and ice-rafted clasts, in combination with published biome data, indicate near-freezing winter conditions. However, paleotemperatures calculated from oxygen isotope data are too warm unless the δ18O value of seawater approached −4‰. Associated stenohaline brachiopods and lack of covariance with δ13C argue against significantly reduced salinity, and published δ18O data from across Australia suggest that conditions recorded by this bivalve were typical of the Gondwanan high latitudes. The negative δ18O value of seawater implied by these data reflects in part the expected meridional gradient in marine seawater composition related to latitudinal variation in evaporation and precipitation in Permian oceans, but is also consistent with previous suggestions of a secular increase in the oxygen isotopic composition of ocean water through the Phanerozoic.