SummarySymptoms of infection by the fungal pathogenGaeumannomyces graminiswere studied in roots of wheat seedlings grown in sand culture in order to assess resistance. Three sets of wheat were studied, 10 cultivars previously thought to differ in infection, and 44 and 54 segregating families from two crosses between four of these wheats, Aus1080 × Condor and Termu89–72 × Bayonet respectively. The isolates ofG. graminisemployed had previously been selected for differences in virulence.The interpretive value of symptoms of infection were considered, by comparison with direct measurement of the spread of hyphae from a fixed point of inoculation, and by repeatability of measurements over experiments. Two symptoms of infection, cortical browning and stelar blackening, are thought to be host mechanisms to resist hyphal invasion and were suppressed by rapid colonisation of root tissues. A third symptom, general discolouring of tissue, appeared to be a necrotic symptom of aggressive colonisation.Since only root segments distal to the point of infection were studied, stelar blackening appeared the least flexible measure of severity of infection. Cortical browning appeared to be influenced by nutritive status of seedlings, and was negatively associated with the incidence of necrotic discolorations. Wheats differed consistently and mostly in cortical browning, and while also differing in extent of necrotic discoloration, difficulty in quantifying the latter character limited its value in assessing resistance toG. graminisin this st