Boreal Sphagnum-dominated peatlands are known to emit isoprene in amounts comparable to emissions from boreal Picea abies-dominated forests. Due to its high reactivity, isoprene can affect the local climate. Increases in isoprene emission under elevated ozone concentration have been detected in a few plant species, but the effects at the ecosystem level are largely unknown. We investigated the isoprene emission from microcosms representing a boreal Sphagnum fen under long-term realistically elevated ozone concentration in open-field conditions. The vegetation of the microcosms was dominated by Sphagnum papillosum Lindb. moss with dense Eriophorum vaginatum L. growth. The mean standardised emissions of isoprene under ambient and elevated ozone concentrations in the growing season of 2005 were on average 1.016 mg m - 2 h - 1 , and 1.308 mg m - 2 h - 1 , respectively. In August 2004, they were 0.434 mg m - 2 h - 1 under ambient, and 0.509 mg m - 2 h - 1 under elevated ozone concentration. Elevated ozone concentration had no statistically significant overall effects on the isoprene emission, although isoprene emissions tended to vary more and were slightly higher under ozone exposure during the warmest weather conditions. The results are in accordance with earlier observations of substantial isoprene emissions from boreal peatlands. As isoprene emissions from peatlands can be increased by ozone during warm periods, further research addressing this issue is needed.