Thaxtomins are unique 4-nitroindol-3-yl containing dioxopiperazines that cause dramatic plant cell hypertrophy and seedling stunting. This family of phytotoxins is produced by Streptomyces species that cause diseases of root and tuber crops; its members are essential for pathogenicity. The symptoms produced by thaxtomin A suggest several potential plant cell targets including the plasma membrane, various components of the cytoskeleton and the cell wall. Dramatic increases in cell volume in onion seedling hypocotyls, radish seedling hypocotyls and tobacco suspension cultures, in response to 0.05–1.0μM thaxtomin A, suggested that this phytotoxin is interacting with one or more conserved plant cell targets. Onion root tip cells treated with thaxtomin A concentrations at or below that which inhibited onion root growth were binucleate or had abnormal cell plates. Thaxtomin A (1.0–3.0μM) inhibited normal cell elongation of tobacco protoplasts in a manner that suggested an effect on primary cell wall development. In summary, these data suggest that thaxtomin A alters, either directly or indirectly, the deposition or composition of monocot and dicot plant cell walls in ways that affect the wall integrity and the ability of the cell to progress normally through cytokinesis.