Intracellar oxalate decarboxylase (ODC, EC 4.1.1.2) activity was screened in the mycelium of 12 white rot fungi. ODC activity was detected in the mycelial extracts of Dichomitus squalens, Phanerochaete sanguinea, Trametes ochracea, and Trametes versicolor (strain R/7) after addition of 5-mM oxalic acid to the liquid culture medium. In D. squalens, intracellular ODC activity increased six-fold with addition of oxalic acid. Production of extracellular organic acids by the four ODC-positive fungi was followed in liquid Cultures and in solid state cultures of spruce wood chips by using HPLC and capillary zone electrophoresis (CZE), The four ODC-positive fungi secreted oxalic acid both in liquid and solid state cultures showing different production patterns until the end of growth (31 days), Upon Cultivation on solid spruce wood chips, manganese peroxidase (MnP) activity peaked simultaneously in these fungi with the accumulation of extracellular oxalic acid. In addition to oxalic acid, glyoxylic and formic acids were detected in the cultures of D. squalens.