Four Korean rye populations with B chromosomes at frequencies ranging from 15 to 55 per cent have been analyzed in two successive generations in a field experiment. B chromosome frequencies were maintained in spite of the environmental change from Korea to Madrid. Values of variables with effect on viability and fertility have been estimated in plants with and without Bs, in both generations. B chromosomes did not affect viability, while fertility decreased when the number of Bs increased. Our data are against an adaptive nature of B chromosomes. First pollen mitosis was observed in 2B plants. The frequency of metaphases with 1B and the rate of nondisjunction at anaphase were high and similar in all populations. Since the decrease of fertility due to Bs and the cytological behaviour of Bs at gametogenesis were similar in all populations, the balance between these two factors cannot account for the establishment of equilibria at different frequencies.