AbstractVarious life history characteristics were studied and compared for 27 species of the genusTrilliumgrowing in Japan and eastern North America, in order to clarify the modes of life history, evolution within this genus. The 27 species showed notable differences in several life history traits, such as population structures and reproductive characteristics, including reproductive allocation, ovule number, reproductive output, propagule size, etc. Remarkable differences in life history traits were observed between pedicellate‐ and sessile‐flowered species. The pedicellate‐flowered species showed relatively larger individual biomasses, larger reproductive outputs, lower energy investment to a single propagule (RA), and smaller seeds. On the other hand, sessile‐flowered species were characterized by smaller individual biomasses, lower reproductive outputs, higher RA, and larger seeds. The results of the principal component analysis (PCA) on the reproductive traits clearly revealed that the morphological and geographical differentiations in the genusTrilliumwere very closely related to the modes of differentiation in significant life history and demographic parameters.Of the various reproductive characteristics, differences in ovule number, reproductive output, seed setting rate, and seed weight clearly reflect the degree of differentiation in these significant fitness components of each species. In spite of the similarity in floral morphology, Japanese erect‐flowered species showed much larger ovule numbers and larger reproductive outputs than those of eastern North American erect‐flowered species. The declinate‐flowered species differed from erect‐flowered species in having lower seed‐setting rates. Furthermore, several sessile‐flowered species with vegetative reproduction exhibited lower ovule numbers and lower seed‐setting rates. These species with vegetative reproduction showed similar convex‐shaped population structures, while sexual reproducing species possessed population structures of a typical sigmoid type. Vegetative reproduction in the genusTrilliumseems to have differentiated to supplement the inefficiency of offspring recruitment by sexual reproduction in ecologically somewhat unstable habitats, such as flood plains of rivers. However, these sessile‐flowered species showed higher relative energy allocations to single propagules (RA), suggesting that the energy cost of producing seeds proportionately increases in harsher and unstable habitats. The results of the present study clearly demonstrate that life history and demographic parameters of the species in the genusTrilliumhave differentiated in direct relation to different underlying environmental regimes peculiar to each s