Extracellular polygalacturonase (PG) production was estimated in vitro, using liquid cultures of three species of brown-rot decay fungi (Postia placenta, Gloeophyllum trabeum and Serpula incrassata), by cup-plate assay, assay of reducing sugars, and decrease in viscosity. Although all three experimental assays demonstrated that PG was induced by pectin in all three fungi, decrease in viscosity gave the best correlation with decay capacity in soil block tests. PG activity, determined as an increase in reducing sugar activity, was greatest in G. trabeum and weakest in S. incrassata. The optimum pH for PG activity was between pH 2.5 and 4.5. Oxalic acid production was also enhanced by pectin and functioned synergistically with PG activity. We conclude that these fungi produce PG that is best induced by pectin and that PG activity exceeds production of xylanase and endoglucanase activity in vitro. Polygalacturonase is likely to act synergistically with oxalic acid to solubilize and hydrolyse the pectin in pit membranes and middle lamellae. Thus, production of PG and oxalic acid should facilitate early spread of hyphae and enhance the lateral flow of wood-decay enzymes and agents into adjacent tracheids and the wood cell wall, thus initiating the diffuse decay caused by brown-rot fungi.