AbstractThe stage class structures and the reproductive characteristics as well as energy allocation to reproductive activities were studied in four North AmericanTrilliumspecies (Liliaceae)—T. grandiflorum, T. erectum, T. undulatum and T. sessile.In addition, a comparison was also made of the life history characteristics of these four North American species with those of Japanese species.As a result, the stage class structures of these four species were discriminated for each species based on leaf area. The study revealed that they all possess similar depletion curves characterized by a conspicuous decrease of individuals belonging to the juvenile stages (from stage class 0 to 2). The switchover of stages, i.e, from one‐leaf to three‐leaved sterile stages, from three‐leaved sterile to fertile stages, orvice versa, was found to occur in different size ranges for each species. In all four species examined, reproductive allocation to total reproductive structures (RA) at the mature fruiting stage was almost independent of biomass, but the number of propagules produced per plant (Pn) was clearly dependent on the size of plants (individual biomass). Three species,T. grandiflorum, T. erertumandT. sessile, showed a proportional increase in Pnin response to the increase in RA, as found in four JapaneseTrilliumspecies, and also in a number of other perennial herbs growing in closed, stable and predictable environments with an outbreeding system. This trend, however, was not obvious inT. undulatum, i.e., Pnwas independent of RA. With respect to seed characteristics, these species exhibited relatively high seed setting rates per ovule.Trillium undulatumshowed very high fecundity, 92.40% and 85.38% in two years' samples, suggesting that this species is a typical inbreeder. Contrary to JapaneseTrilliumspecies, which showed proportional increase in seed weight in response to an increase in ploidy levels (from 2xto 6x), these diploid species showed wide variation in seed weight, ranging from 4.15 mg to