A form of the take-all fungus ( Gaeumannomyces graminis) which attacked oats in sterile sand tests, was isolated from 59% of 86 randomly sampled oat crops in Western Australia, and from 39% of 114 G. graminis-infected wheat and barley crops. Twenty-nine per cent of 251 isolates from wheat and barley crops were pathogenic to oats. Oat-attacking isolates were more prevalent in the southern cereal-growing area, where oats are more frequently grown, than in the northern area. In contrast to those isolates classed as non-pathogenic to oats in the sterile sand tests, those classed as pathogenic and causing significant top-weight reduction in barley in artificially inoculated pot and field experiments also caused significant top-weight reduction in oats. In pot experiments, top-weight reduction in barley was greater than in oats, but the reverse was true for most isolates in the field experiment. Ascospore length of isolates was related to ability to attack oats and not to the original cereal host from which the isolates were obtained, but few isolates were in the range of G. graminis var. avenae.