Polyploidy is often thought to be associated with asexual reproduction, self-compatibility and self-pollination. Polyploids are also often regarded as being less sensitive than diploids to the negative effects of inbreeding. The reproductive biology of two lilioid species, the diploid Anthericum ramosum and the tetraploid A. liliago, was compared in a series of greenhouse and field experiments. Pollination experiments were carried out to study agamospermy (seed set without fertilization), self-compatibility and early inbreeding depression for both species. Tetraploid A. liliago showed no greater ability to self-pollinate than diploid A. ramosum and neither of the species showed agamospermy. Both species were self-compatible, but levels of fruit and seed set were lower after self-fertilization than after cross-fertilization in A. ramosum (P<0.001). No reduction in fecundity after self-fertilization was observed in A. liliago. Fruit set in natural populations of A. ramosum increased significantly (P<0.001) after emasculation (which excluded the possibility of autodeposition of pollen within flowers). In contrast, emasculation of flowers reduced the fruit set within natural populations of A. liliago (P<0.05). The reproductive biology of diploid A. ramosum and tetraploid A. liliago is discussed in the context of polyploidization and the reduced fruit and seed set after self-fertilization in A. ramosum is discussed in terms of early inbreeding depression.