The presence of jasmonic acid (JA) was searched in the bacteriumEscherichia colinon-pathogenicRC424strain and in the pathogenicRC-7strain. In addition, JA was screened in plant bacteria such asAzospirillum sp., Bacillus cereus, Bradyrhizobium sp. GB2andP53, Rhizobium meliloti, and in their respective culture media. Our results demonstrated that both strains ofEscherichia colisynthetize jasmonic acid and release this compound to the culture medium. Jasmonic acid was also identified as an endogenous compound of yeast (Saccharomyces cerevisiae) used in the medium. The fatty acid linolenic acid, precursor of JA in higher plants, was not found, suggesting a different biosynthetic pathway in these microorganisms. By contrast, other bacteria metabolized the jasmonic acid supplied by the yeast to the culture medium.