Two small M-2 populations, consisting of 39 and 50 plants, respectively, obtained by EMS-mutagenesis of an inbred line derived from oilseed rape cv. Linetta, were screened for altered leaf response to artificial inoculation with Sclerotinia sclerotiorum. In both experiments, the M-2 population exhibited greater variation and a lower mean infection value than the parental population; individuals in the most resistant class were obtained only from the M-2 population. Parent-progeny analysis of disease response scores revealed significant regressions only for the mutagenised population, with narrow-sense heritabilities of 0.75-0.83, compared to 0.14-0.22 for the parental population. When larger populations (approximately 600 individuals per population) were screened, similar results were obtained. Mutants with significantly greater resistance than the most resistant 'Linetta' line were obtained at frequencies of 1.7% (from an M-2 population size of 593) to 5.1% (n = 39). The altered leaf response to Sclerotinia in selected mutant lines was positively correlated with stem response to artificial inoculation. Detailed analysis of one mutant (HH-1), with significantly higher Sclerotinia resistance than the parent, demonstrated that HH-1 was more resistant to artificial stem inoculation than four commercial varieties tested, including cv. Briol, which is reported to exhibit high levels of resistance in the field. Field trials in moderately- and heavily-infested soils showed that HH-1 exhibited significantly greater resistance to natural infection than 'Linetta', with percentage plant deaths of 5.3% (compared to 22.4% in the parental population) and 13.6% (47.3%) under moderate and high inoculum pressure, respectively. The seed yield of HH-1 was significantly higher than that of the parent population under a heavy Sclerotinia infestation; in the absence of Sclerotinia, the yield difference between the two populations was not significant. The implications of these results are discussed in respect of a re-evaluation of the efficacy of mutagenesis for the isolation of agronomically valuable micro-mutants.