An allotriploid (ALA, 2n=3x=36) BC₁ plant was obtained by backcrossing a diploid F₁ interspecific hybrid (LA, 2n=2x=24), derived from a Lilium longiflorum (L genome) and an Asiatic hybrid (A genome), to the latter parent. This allotriploid was backcrossed to a diploid Asiatic hybrid (2n=2x=24) and to an allotetraploid (LLAA, 2n=4x=48) LA hybrid. A total of 25 plants of these crosses were examined for ploidy level, and 12 individuals were analyzed for their genome constitution through genomic in situ hybridization (GISH). In most cases the progenies from the triploid-diploid (3x-2x) crosses consisted of aneuploids. Further more, there was evidence for the formation of near-haploid (x=12+2) to triploid (3x=36) gametes in the allotriploid BC₁ plant. The progenies of triploid-tetraploid (3x-4x) cross also consisted of mostly aneuploids but in this case the triploid female parent had contributed predominantly near-triploid (2n) gametes for the origin of BC₂ progenies. The different ploidy levels observed between 3x-2x and 3x-4x crosses are possibly caused by preferential fertilization or survival resulting in a different ratio of chromosome numbers between the embryo and endosperm. Though Lilium has a tetrasporic, eight-nucleate type of embryo sac formation (Fritillaria type), the observed difference between the progeny types in 3x-2x and 3x-4x crosses is comparable to that of observed in monosporic eight nucleate types (Polygonum type) that predominate in most genera of Angiosperms. An important feature of the genome constitution of the progenies was that the homoeologous recombinant chromosomes were transmitted intact from BC₁ to BC₂ progenies in variable numbers. In addition, there was evidence for the occurrence of new homoeologous recombinations in the triploid BC₁. Of the two euploid BC₂ plants one had originated through the parthenogenetic development of a 2n egg and the other had originated through indeterminate meiotic restitution (IMR).