A cell‐free culture filtrate of the plant growth inhibitory bacterial isolate Å313, identified asPseudomonas fluorescens,was tested for its effect on wheat root elongationin vitro,with and without various pretreatments. The filtrate showed a strongly inhibitory effect on root elongation and could be heated to 100°C for 5 min or incubated at a pH within the range of 4–10 without losing its activity. Unlike other root growth‐inhibitory bacterial metabolites the effect of the filtrate was not reversed by methionine, nor was any inhibitory activity present in the water phase after extraction with chloroform. The inhibitory agent(s) was formed when the bacterium was grown in an artificial medium as well as in root exudates from wheat. Two wheat cultivars differing in reaction to inoculation with living cells of Å313 showed the expected difference in growth reduction when exposed to culture filtrate, indicating a cultivar difference in sensitivity to the metabolite(s) formed by the bacterium. Isolate Å313 invaded intercellular spaces of the root cortex of gnotobiotically grown wheat plants, but did not produce pectolytic enzymesin vitro,nor induce a hypersensitive response in tobac
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