Evaluation with the pot assay at seedling stage in greenhouse showed that oat (Arena saliva) 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 u