The effects of increased nitrogen influx on Sphagnum growth and on interspecific competition between Sphagnum species were studied in a 3-yr experiment in mires situated in two areas with different rates of airborne N deposition. Sphagnum growth was recorded after various supplementary N influxes (0, 1, 3, 5 and 10 g m −2 yr−1) in hummocks and lawn communities. Sphagnum biomass production decreased with increasing N influx in both areas. After the first season at the low-deposition site, Sphagnum showed an increased growth in length with the intermediate N treatment, but in the second and third seasons the control treatment had the highest growth in length. Capitulum dry mass increased with increasing N influx. Sphagnum N concentration and N/P quotient were higher at the high- than at the low-deposition site. The low quotient at the low-deposition site, together with the initial growth increase with intermediate N supplements, indicates that growth was N-limited at this site, but our lowest N supplement was sufficient to reduce growth. The N treatments had no effect on interspecific competition between the Sphagnum species. This indicates that the species have similar responses to N. The species studied all occur naturally on ombrotrophic, N-poor sites and show low tolerances to increased N influx. Reduced Sphagnum production may affect the carbon balance, changing the mires from C sinks to sources.