The involvement of pectin and pectinases in the host pathogen interaction of P. terrestris and Allium cepa were investigated by use of pectin histochemical stains at the electron microscope level. Several different pectin histochemical stains were used, but only two (the hydroxylamine reaction, and the pectin stain developed in this study) gave reliable results. It was observed that by the time the fungus has reached the host cell wall pectinesterase and endoplygalacturonase have fully removed the pectin from the host cell wall. When this has occurred, cellulases are released which continue to degrade the host cell wall. Electron transparent areas appeared where there were no COOH groups.