The growth of interspecific hybrids between oilseed rape (Brassica napus ssp. oleifera) and hoary mustard (Hirschfeldia incana) was studied in different conditions of temperature and photoperiod in individual pots and in competitive conditions with each of the two parental species.  In the absence of competition, the hybrid plants were intermediate between parents for morphological and developmental traits.  Under competitive conditions, the hybrids were closer to hoary mustard.  The results of the studies indicated that hybrids and hoary mustard would not be better competitors against oilseed rape, but hybrids would be more competitive in a hoary mustard population.  The relevance of these studies to the environmental impact of the release of transgenic crops is discussed.  Hybrid survival would not limit gene escape through interspecific hybridization.