Ocean acidification driven by rising levels of CO 2 impairs calcification, threatening coral reef growth. Predicting how corals respond to CO 2 requires a better understanding of how calcification is controlled. Here we show how spatial variations in the pH of the internal calcifying fluid (pH cf) in coral (Stylophora pistillata) colonies correlates with differential sensitivity of calcification to acidification. Coral apexes had the highest pH cf and experienced the smallest changes in pH cf in response to acidification. Lateral growth was associated with lower pH cf and greater changes with acidification. Calcification showed a pattern similar to pH cf, with lateral growth being more strongly affected by acidification than apical. Regulation of pH cf is therefore spatially variable within a coral and critical to determining the sensitivity of calcification to ocean acidification.