1.Nothoscordum fragranshas colonised a wide range of habitats and, once introduced, can spread to become a weed.2.In a clone of this species with a heterozygous complement (2 n=19=13L + S) similar to those described by previous investigators, the ovules contained more than one embryo. After fertilisation, an embryo with a suspensor could be identified in some embryo-sacs. Usually, after competition for food reserves, only one seedling from each seed reached maturity, and this was frequently from an adventive nucellar embryo.3.Non-disjunction and lagging of unpaired univalents at meiosis, particularly S chromosomes of the heterotrivalent, result in many aneuploid pollen grains, but about 50% of pollen tubes were capable of reaching the ovule. A weak infertile aneuploid (2n +1 = 20 = 14L + 6S) was raised by artificially separating seedlings produced by one seed.4.None of the expected aneuploid or structurally homozygous products of sexual reproduction survive under natural conditions.5.It was concluded that vegetative and apomictic reproduction affects the rapid spread of genotypes produced by sexual reproduction. A system involving balanced lethals and polyembryony eliminates all but the best adapted, heterozygous individuals in this species with enforced inbreeding. Structural heterozygosity is not an essential feature of this mechanism, but as it involves nucleolar chromosomes, is perhaps preserved for its effect on chromosome activity.