Plants with self-incompatibly (SI) frequently exhibit variable expression of this trait. The study reported here investigates the breakdown of SI in a perennial bellflower (Campanula rapunculoides) using a standard quantitative genetics approach to examine the relative influences of genotype, prior fruit-set and floral age on seed-set from self-pollinations with standardized pollen loads. Cross-pollen was used on separate flowers for comparison. The results obtained indicate that genotype (clone) explained a significant amount of the total variation and plants with few developing fruits showed stronger expression of SI on young flowers, and weaker expression of SI on old flowers than plants with many fruits (fruit-by-floral-age interaction, P<0.02). A second experiment determined that the stigmatic curling accompanying floral age does not influence expression of SI. A significant clone-by-floral-age interaction suggests that continuous variation in self seed-set of putatively SI species may be the result of genotype-by-environment interactions. It is concluded that SI is a phenotypically plastic trait in C. rapunculoides and its breakdown responds to conditions that are indicative of low pollinator activity.