The oxalic acid metabolism of Postia placenta was studied in liquid culture in order to identify the metabolic pathways of biosynthesis and to determine whether oxalic acid physiology could be correlated with efficiency in wood decay. Oxalic acid production was determined for test strain MAD698 grown in a basal defined medium with a variety of different carbohydrate and nitrogen sources. The highest quantity of oxalic acid was formed in the presence of glucuronate; lower quantities were formed in response to glycolic acid and glyoxylate, suggesting that the predominant pathway of synthesis for oxalic acid is the glyoxylate bypass of the tricarboxylic acid cycle. Little or no oxalic acid was formed when simple sugars served as the carbon source. Ammonium phosphate (monobasic), ammonium chloride, ammonium sulphate, and peptone also stimulated oxalic acid production. An aberrant strain Postia placenta (ME20) that fails to cause significant weight loss in wood produced ten times less oxalic acid in response to glucuronate than MAD698. Strain ME20 also failed to form detectable concentrations of oxalic acid when glycolic acid or glyoxylate were the sole sources of carbon. The inability of ME20 to cause weight loss in wood may be related to a decreased efficiency or defective regulation of the biosynthetic pathway for oxalic acid.