Recently, field and laboratory studies have demonstrated the significant impact of variations in seawater [CO2] and related changes in carbonate chemistry on both planktic and benthic marine calcifying organisms. (e.g. Bijma et al., 1999; Kleypas et al., 1999; Leclercq, 2000 et al., Riebesell et al., 2000; Zondervan et al., 2001; Delille et al., 2005; Gazeau et al., 2007). For foraminifera a model has been proposed to explain the calcification process (Bentov and Erez, 2005, 2006): Seawater is taken up via endocytosis and transported to the site of calcification. During transport the composition of the solution inside the vesicle is modified (concentrating Ca2+ while removing other divalent cations, notably Mg2+, and at the same time pumping H+ to acidic vesicles, thereby increasing pH and CO32- in the calcification vesicles). Because the calcification is based on ambient seawater, foraminifers are among the best recorders of paleo-proxies but it also explains why foraminifera are so sensitive to ocean acidification. A similar model has been proposed for corals (Erez, personal com.). The similarity in the ultra-structures of different calcifying organisms also suggests a common ancestral calcification mechanism. We report in more detail on the impact of ocean acidification on planktonic and benthic foraminifera, through controlled laboratory culture experiments. The impact was monitored by weighing shells and using SEM and AFM techniques. The overall aim of this study is to develop/improve a process based understanding of biocalcification in foraminifera which would allow predicting how these calcifiers cope with an acidifying ocean and which consequences this has for the structural properties of their skeletons.