Overlap between flowering of oilseed rape (Brassica napus var. oleifera Metzger) and wild mustard (Sinapis arvensis L.), artificial hybridization between the two species, spontaneous crosses, and backcrossing were assessed to estimate the risk of escape of genes from transgenic crops towards the wild species.  In the Burgundy region of France, wild mustard flowers later than oilseed rape.  Exposure to cross pollination was two to five times greater with late-flowering cultivars than with early cultivars.  Artificial hybridizations using in vitro ovary culture produced up to 1 seed per 100 pollinated flowers.  No hybrid was found among 2.9 million seeds produced by wild mustard grown in a garden in presence of a herbicide-resistant transgenic cultivar.  No more than six hybrids were obtained from 50 000 flowers of a male-sterile oilseed rape grown in presence of wild mustard.  Artificial hybrids grown in presence of wild mustard, or hand-crossed, produced a few aborted seeds.  Thus, in similar 'normal conditions', it may be concluded that a flower of these two species has a probability smaller than 10 -10 of having an interspecific hybrid progeny.