Pseudomonads, which inhibit root extension, can be present in the winter wheat rhizosphere in large numbers, but they are not detectable until late winter or early spring. Their presence was not related to the presence of wheat straw residues or type of tillage, although they were present on the wheat residues when they appeared in the rhizosphere. Wheat seedlings were more sensitive to the bacteria at 15° C than at 20° C during bioassays. The type of agar used in the bioassay can affect the results obtained. The inhibitory factor expressed by the pseudomonads is quite variable and is radically affected by transfer of isolates.