Manganese (Mn) toxicity limits plant growth in acid soils. Although Mn toxicity induces oxidative stress, the role of superoxide dismutase (SOD, EC.18.104.22.168) isoforms in conferring Mn tolerance remains unclear. Seedlings of ryegrass cultivars Nui (Mn-sensitive) and Kingston (Mn-tolerant) were hydroponically grown at 2.4 (optimal) or 750μM Mn (toxic) concentration, and harvested from 2 to 48h. Kingston showed higher shoot Mn than Nui at 2.4μM Mn. At toxic supply, shoot Mn concentration steadily increased in both cultivars, with Kingston having the highest accumulation at 48h. An early (2h) increase in lipid peroxidation under Mn excess occurred, but it returned (after 6h) to the basal level in Kingston only. Kingston exhibited higher SOD activity than Nui, and that difference increased due to toxic Mn. In general, Mn-induced gene expression of Mn- and Cu/Zn-. SOD isoforms was higher in Nui than Kingston. Nevertheless, under Mn excess, we found a greater Fe-SOD up-regulation (up to 5-fold) in Kingston compared to Nui. Thus, Fe-SOD induction in Kingston might explain, at least partly, its high tolerance to Mn toxicity. This is the first evidence that Mn toxicity causes differential gene expression of SOD isoforms in ryegrass cultivars in the short-term. © 2013 .