Initial observations on Nicotiana rustica suggested that the percentage of seed set by outcrossing might vary considerably in a segregating population and that this might be due to variation in the position of the stigma relative to the anthers in the corolla-tube. This, measured as the difference in level between the stigma and the ring of anthers, is termed heterostathmy and the average expression of this character could be significantly altered by a short programme of selection. These structural modifications were accompanied by developmental modifications affecting the timing of anther dehiscence relative to the opening of the flower. Tests indicated that these changes significantly affected the rate of outbreeding.