Tan spot is an important leaf-spotting disease of wheat which has become more prevalent due to the adoption of conservation tillage practices. Certain isolates of the ascomycete 'Pyrenophora tritici-repentis', the causal agent of tan spot, are known to produce host-selective toxins which are involved in pathogenesis. PtrToxA, which induces tan necrotic lesions in the leaves of sensitive wheat genotypes, was the first of these toxins to be characterized, and is a 14 kDa ribosomally-synthesized polypeptide. The purpose of this study was to examine the effects of PtrToxA on sensitive wheat leaf tissue using light, fluorescence, and transmission electron microscopy. The microscopic effects of the toxin were also compared to those of freezing and thawing, desiccation of detached leaves, and plasmolysis at the time of fixation. The effect of PtrToxA on water relations, cell walls, and tonoplast are discussed with respect to the potential site and mode of action of this toxin in sensitive wheat leaf tissue. (Abstract shortened by UMI.)