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Evaluation with the pot assay at seedling stage in greenhouse showed that oat (Avena sativa) was highly resistant to take-all disease to which, however, wheat (Triticum aestivum) was extremely susceptible. The oat roots were shown to be inhibitory to the invasion and spread of take-all causing fungus G. graminis var. tritici by the following criteria: ( i ) less infection sites were observed (about 1/7 of those in wheat); (ii) the ectotrophic growth of G. graminis var.tritici on oat roots was much slower than that on those of wheat, and the runner hyphae appeared as kidney- or fork-shaped hyphopodia on the surface of oat roots which could not be discerned on that of wheat roots; (iii) the period from inoculation to penetration into the epidermis of oat roots was about 2.9 times as long as that of wheat; (iv) the infection hyphae were hindered substantially when it was about to penetrate into the epidermis of oat roots with the mycelium deformed; and ( v ) the cortical layer of oat roots was revealed to be unsuitable for the G. graminis var. tritici infestation as some lysed hyphae were found therein, and the spread of hyphae from the first layer of cortex to the pericycie needed 108 h, about 1.8 times as long as it did on wheat roots.