Oilseed rape has become an important weed in arable rotations and a feral plant of field margins, soil dumps and roadsides. Seed persistence in the soil following induced secondary dormancy is thought to perpetuate these weed and feral populations. Potential variation between cultivars has been suggested in previous work in the extent of secondary dormancy and other germination traits, and in underlying genetic heterogeneity. The aim of this work was to quantify in more detail inter- and intra- cultivar variation in germination traits for six oilseed rape cultivars, to confirm that variation was consistent in laboratory and field, and to ascertain whether there is a genetic base to this variation. The cultivars are Askari, Bristol, Gazelle, Libravo, Martina and Rocket, selected on the basis of their suspected heterogeneity. Laboratory germination tests were conducted at 4°C, 10°C and 19°C on a thermal plate and confirmed substantial inter-cultivar variation in germination rate, induction of secondary dormancy and the temperature stimuli required for dormancy break. The phenotypic traits were quantified by mathematical parameters and cultivars ranked in order of decreasing heterogeneity. DNA analysis was made on leaf tissue of early, middle and late germinating phenotypes using two simple sequence repeat primers. There was heterogeneity and phenotypic variability generally, but a direct association between phenotype and genotype was found only in the cultivar Martina. Field emergence trials revealed non-linearity in emergence and a strong similarity between laboratory germination and field emergence curves. Cultivar heterogeneity was found to be similar for emergence rate and post-winter emergence. Again there was evidence of an association between heterogeneity in emergence and genetic heterogeneity in the DNA markers. The results suggest that standard seed testing should be carried out at low temperatures to detect any hidden variability in germination. Plant breeders should be cautious about introducing variability into new breeding lines as this might increase the potential persistence of feral populations and the risk of gene transfer to later.