Anther-stigma separation (herkogamy) is highly variable within populations of Mimulus ringens, a bumblebee-pollinated perennial herb with a mixed-mating system. The relationship between this floral trait and individual outcrossing rates was studied in two experimental populations composed of genets with unique multilocus combinations of homozygous genotypes. This facilitated determination of individual outcrossing rates through unambiguous assignment of paternity to all 1560 sampled progeny. In each population there was significant heterogeneity among maternal families in frequencies of self and outcross progeny. Individual outcrossing rates were positively correlated with anther-stigma separation.