Description: In Palau, calcification rates of two reef-building coral genera (Porites and Favia) are maintained across a strong natural gradient in aragonite saturation state (Omega ar) ranging from 3.7 to 2.3. This observation contrasts the strong sensitivity to decreasing ?ar that these genera demonstrate in both laboratory CO2 manipulation experiments and in field studies. Moreover, in contrast to other naturally more acidic coral reefs, benthic communities in Palau's low-Omega ar (Omega ar = 2.3) Rock Island reefs display ecological indices consistent with healthy communities. A laboratory CO2 manipulation experiment and a field-based reciprocal transplant were used to investigate whether the apparent lack of sensitivity to ocean acidification of Palau's Porites corals can be attributed to local adaptation to chronic acidification or to environmental factors that allow corals to thrive despite extreme pH conditions. In a two-month laboratory incubation, calcification rates of Palau Porites from both environments were insensitive to changes in Omega ar over the range 1.5 to 3.0, suggestive of an adaptive, rather than environmental, mechanism for acidification tolerance. However, in the reciprocal transplant, corals transplanted between reefs at different ambient ?ar levels showed significant declines in calcification rates and high mortality, while corals returned back to their reef of origin were alive after 17 months in the field. Interpreted within the framework of the experimental result, the failure of pH/Omega ar-tolerant corals to successfully transplant between different reef sites hints at local adaptation to other (non-pH) environmental factors such as light, temperature, and/or flow that co-vary with Omega ar across Palau's natural acidification gradient.