Stenocarpella maydis, a fungal pathogen of maize, produced polygalacturonases (PGs) when grown on pectin or maize cell walls. An extract from bean (Phaseolus vulgaris L.) which contained an active inhibitor of Aspergillus niger PG, also inhibited S. maydis PG in a reducing sugar assay. Bean polygalacturonase inhibitor protein (PGIP) was purified from the extract by affinity chromatography, and this inhibited the S. maydis PG in reducing sugar assays and an overlay gel activity assay. Inhibition was abolished by boiling of the PGIP. Since purified PGIP could still be a mixture of PGIPs with similar physical properties but different inhibitory activities on different fungal PGs, the bean pgip-1 gene was cloned and expressed in transgenic tomato. PGIP extracts from the transgenic tomato inhibited both A. niger and S. maydis PG, whereas extracts from a control untransformed tomato did not. This indicated that bean PGIP-1 is able to inhibit the S. maydis PGs.