Warming and changes in ocean carbonate chemistry alter marine coastal ecosystems at an accelerat-ing pace. The interaction between these stressors has been the subject of recent studies on reef organisms such as corals, bryozoa, molluscs, and crustose coralline algae. Here we investigated the combined effects of elevated sea surface temperatures and pCO 2 on two species of photo-symbiont-bearing coral reef Foraminifera: Heterostegina depressa (hosting diatoms) and Marginopora vertebralis (hosting dinoflagellates). The effects of single and com-bined stressors were studied by monitoring survivorship, growth, and physiological parameters, such as respiration, photochemistry (pulse amplitude modulation fluorometry and oxygen production), and chl a content. Specimens were exposed in flow-through aquaria for up to seven weeks to combinations of two pCO 2 (*790 and *490 latm) and two temperature (28 and 31 °C) regimes. Elevated temperature had negative effects on the physiol-ogy of both species. Elevated pCO 2 had negative effects on growth and apparent photosynthetic rate in H.depressa but a positive effect on effective quantum yield. With increasing pCO 2 , chl a content decreased in H. depressa and increased in M. vertebralis. The strongest stress responses were observed when the two stressors acted in combination. An interaction term was statistically signifi-cant in half of the measured parameters. Further explora-tion revealed that 75 % of these cases showed a synergistic (= larger than additive) interaction between the two stressors. These results indicate that negative physiological effects on photosymbiont-bearing coral reef Foraminifera are likely to be stronger under simultaneous acidification and temperature rise than what would be expected from the effect of each of the stressors individually.