Abstract: Here we show that P. ananatis produces at least three phosphonates, two of which were purified and structurally characterized. The first, designated pantaphos, was shown to be 2-(hydroxy(phosphono)methyl)maleate; the second, a probable biosynthetic precursor, was shown to be 2-(phosphonomethyl)maleate. Purified pantaphos is both necessary and sufficient for the hallmark lesions of onion center rot. Moreover, when tested against mustard seedlings, the phytotoxic activity of pantaphos was comparable to the widely used herbicides glyphosate and phosphinothricin. Pantaphos was also active against a variety of human cell lines.