Difficulty is 'sometimes experienced in distinguishing between O. graminis and sterile fungi which form "Ophiobolus-like" runner hyphae on cereal roots. However, it is possible that nuclear distribution in mycelium may prove a differential character as different distributions have already been reported for O. graminis (Chambers and Flentje 1967) and a cereal root-attacking strain of Rhizoctonia solani (Flentje, Stretton, and Hawn 1963). This paper, therefore, describes comparisons of nuclear distribution in O. graminis and several other cereal root pathogens including a sterile fungus which formed Ophiobolus-like runner hyphae.