The outcrossing rate in the chestnut blight fungus, Cryphonectria (End
othia) parasitica, was estimated in a natural population using data fr
om six unlinked restriction fragment length polymorphism (RFLP) loci a
nd DNA fingerprinting. This fungus was shown to have a mixed mating sy
stem with both self-fertilization and outcrossing in the same populati
on. In a sample of 22 perithecia, all progeny segregated in 1:0, 1:1 o
r 0:1 ratios at RFLP loci; there was no evidence for multiple male par
ents in ascospore families from single perithecia. The multilocus esti
mate of the outcrossing rate was 0.74 for the RFLP data; the proportio
n of perithecia with progeny segregating for fingerprint fragments was
0.73. Vegetative compatibility was not a sensitive marker for detecti
ng outcrossing as only 11 perithecia (50 per cent) showed segregation
with this marker, compared with 16 (73 per cent) for DNA fingerprintin
g. We are not aware of any other report of a mixed mating system of a
fungus in a natural population with an outcrossing rate significantly
different from 1.0 or 0.0.