Precipitation of sparsely soluble calcium oxalate causes scaling problems in the pulp and paper industry. A potential solution is to degrade the oxalic acid using oxalate-degrading enzymes. Four novel fungal oxalate decarboxylases were evaluated in experiments with 16 pH-adjusted bleaching filtrates collected from mills producing mechanical pulp or kraft pulp. The enzymes were also tested in five of the filtrates from mechanical pulping at authentic pH and elevated temperature (55°C). The enzyme that performed best in the screening was selected for a small-scale experiment performed in a mill producing mechanical pulp. The enzyme degraded 70% of the oxalic acid in the fresh filtrate after one hour, without pH adjustment and at the prevailing process temperature (65°C). The new enzyme performed considerably better than the well-studied oxalate decarboxylase from Aspergillus niger, which only degraded 4% of the oxalic acid under the same conditions.