Forty-three Australian cultivars or breeding lines of Brassica napus ( canola, oilseed rape) and 2 cultivars of Brassica juncea (mustard) were inoculated with infective sap containing isolate WA-Ap of Turnip mosaic virus (TuMV), which belongs to TuMV pathotype 8. The types of reactions obtained were: necrotic spots in inoculated leaves without systemic infection (R-N), chlorotic blotches in inoculated leaves without systemic infection (R), and chlorotic blotches in inoculated leaves accompanied by systemic infection that consisted of either necrotic spots (+(N)) or chlorotic blotches (+). The RN and +(N) reactions are consistent with those expected in the presence of 4 strain-specific TuMV resistance genes TuRB01 (+(N) response), TuRB03 (+(N) response) and TuRB04 with TuRB05 (RN), with(+) indicating a susceptible response. However, which resistance gene corresponds to the R response is unclear. The RN (TuRB04 with TuRB05) type of response was the commonest. Only one genotype lacked any TuMV resistance, and segregation for more than one different type of resistance response occurred within 22 genotypes and some segregated for resistance and susceptibility. Some genotypes segregated for all 3 types of resistance response found. The reaction of 2 plants of cv. Rivette was atypical as they developed both necrotic spots in inoculated leaves and systemic chlorotic spots. Since breeding for TuMV resistance is not undertaken in Australia, these results indicate frequent but inadvertent crossing with parental lines carrying TuMV resistance. Widespread occurrence of TuMV resistance genes and the possibility that many Australian TuMV isolates may not be well adapted to B. napus may explain the low incidence of this virus found in Australian B. napus crops.