Two rye populations with a B chromosome polymorphism were subject to a range of environmental stress conditions, by varying the sowing density, to assess the relative fitness of individuals with and without B chromosomes. In the Experimental population, characterised by a wide range and high frequency of B numbers, the B chromosome individuals were found to compete and survive less well than their 0 B counterparts throughout die range of densities used. Much of the reduced fitness in this population was due to the presence of odd-numbered and high-numbered B combinations. Progenies derived from plants surviving to maturity, however, were found to have significantly higher B frequencies, and a greater range of B numbers, than their parent plants. The Korean population, with a much lower B frequency, and composed mainly of 0 B and 2 B types, showed a more complex relationship between B frequency and plant density. At low densities plants with Bs had a selective advantage over those without. They also responded more favourably, in terms of survival, at very high rather than at intermediate densities. In this variety there were no significant differences in mean B frequency between the standard population from which the seed sample originated, the parent plants subjected to density stress and suffering a high mortality rate, and the seed progenies derived from these parents.