The genus Liriodendron L. consists of a southeast Asian-eastern North American disjunct species pair, but the genus had a much wider distribution in the Northern Hemisphere during the late Cretaceous and the Tertiary. Although generally similar in morphology, the two extant species are measurably different. In 1973 and 1977 they were hybridized, and interspecific heterosis was observed in the progeny. After seven years, the interspecific hybrids synthesized in 1973 had a significantly greater biomass than intraspecific hybrids of L. tulipifera. The growth rate of interspecific and intraspecific F2 seedlings was approximately equal. A preliminary survey of flavonoid extracts in L. tulipifera populations showed a high level of quantitative variation that was not obviously correlated with either geographic or environmental parameters. While there was little flavonoid variation in one small L. chinense population, a tree from a second locality in China was markedly divergent in its chromatographic pattern. An isozyme survey of individual trees from a few populations showed L. tulipifera to be moderately heterozygous. Six trees of L. chinense from one locality were identically homozygous, while a seventh tree from a different locality was equally homozygous but for different alleles at a number of loci. The available evidence indicates that the two Liriodendron species have not diverged very far from a presumed common ancestor. However, L. tulipifera is an abundant, almost weedy, species in some parts of its range, whereas L. chinense has suffered from population restriction and loss of heterozygosity to the degree that it shows inbreeding depression.