The response of plant growth to rising CO2 levels appears to depend on nutrient availability, but it is not known whether the growth of bog plants reacts similarly. We therefore studied the effects of elevated CO2 in combination with N supply on the growth of Sphagnum mosses and vascular plants in ombrotrophic bog vegetation. Because the growth of Sphagnum is less nutrient-limited than that of vascular plants, we hypothesized that Sphagnum would benefit from elevated CO2. In our greenhouse experiment, peat monoliths (34 cm diameter, 40 cm deep) with intact bog vegetation were exposed to ambient (350 ppmv) or elevated (560 ppmv) atmospheric CO2 combined with low (no N addition) or high (5 g N m-2 yr-1 added) N deposition for two growing seasons. Elevated atmospheric CO2 had unexpected deleterious effects on the growth of Sphagnum magellanicum, the dominant Sphagnum species. Growth was greatly reduced, particularly in the second growing season when, regardless of N supply, the mosses looked unhealthy. The negative CO2 effect was strongest in the warmest months, suggesting a combined effect of elevated CO2 and the raised temperatures in the greenhouse. High N deposition favored Rhynchospora alba, which became the dominant vascular plant species during the experiment. Biomass increased more when N supply was high. There were no significant effects of elevated CO2 on vascular plants, although elevated CO2 combined with high N supply tended to increase the aboveground vascular plant biomass. As Sphagnum is the main carbon-sequestrating species in bogs and rising atmospheric CO2 levels are likely to be followed by increases in temperature, there is an urgent need for further research on the combined effects of elevated CO2 and increased temperature on Sphagnum growth in bog ecosystems.