Two isoforms of the shikimate pathway enzyme 5-enol-pyruvyl-shikimate-3-phosphate synthase, previously purified from maize cultured cells and both found to be functionally located in the plastid while showing different pattern of expression during the culture growth cycle, were characterized with respect to physical and functional properties. A high degree of similarity was found as to their structural features, with the only exception of a slight difference in molecular mass. Both enzyme activities were extremely susceptible to the inhibition brought about by the herbicide glyphosate, and not subjected to feed-back regulation by aromatic amino acids or shikimate pathway intermediates. A more pronounced difference was evident in thermal stability, and catalytic efficiency as judged from the comparison of catalytic constants, affinities for the two substrates and activation energy values. The isozyme detectable in actively-proliferating cells, when the plant cell demand for aromatic amino acids increases, resulted the more stable and efficient. Data are consistent with the hypothesis of an isoform-based mechanism of enzyme level modulation in plant aromatic metabolism.