Abstract Cornexistin, a microbial product isolated from the fungus Paecilomyces variotii SANK 21086, has postemergence herbicidal activity on several important weeds at rates (0.5 kg/ha) that do not affect maize (Zea mays L.). In a duckweed (Lemna pausicostata Hegelm. 6746) bioassay, cornexistin at 333 nM caused pronounced inhibition of growth within 72 hr. At the ultrastructural level, the first symptom was shrinkage and then disappearance of chloroplast starch grains, followed by plasma membrane and tonoplast disruption. Carbon dioxide fixation, as measured by infrared gas analysis, was reduced in a linear fashion by 10 μM cornexistin, resulting in complete inhibition after 24 hr. Photosynthetic parameters more directly associated with light reactions, P-515 electrochromic transient and variable chlorophyll fluorescence, were more slowly affected by cornexistin. Five hundred micromolars each of L-aspartate, L-glutamate, or the TCA cycle intermediates, succinate, citrate, malate, oxaloacetate, and α-ketoglutarate prevented cornexistin phytotoxicity when these compounds were incorporated into the culture medium at the time of first exposure to the herbicide. D-Aspartate and D-glutamate had little or no effect on phytotoxicity. Sucrose at 15 mM was also very effective in reducing the phytotoxicity of cornexistin, whereas concentrations of 1 mM or lower were ineffective. Aminooxyacetic acid (AOA) at 3.33 μM caused phytotoxicity effects similar to those of cornexistin. AOA effects were overcome by the same compounds that prevented cornexistin phytotoxicity. Cornexistin inhibited aspartate amino transferase (AAT) in vitro by only 20-30% at high concentrations, only after incubation in a cell extract. In vitro incubation of these L. pausicostata protein extracts also caused changes in AAT activity stain patterns on polyacrylamide gels. Cornexistin may be metabolized to an inhibitor of one or more AAT isozymes in vivo.