The sustainability of natural grasslands is in large part dependent upon the successful ingress and propagation of alien plant species. We attempted to characterize the propagation potential of the dominant plant species of a southeastern Montana mixed-grass prairie community including two introduced grasses, Bromus japonicus and B. tectorum. Aboveground standing crops and densities of seeds were estimated on 10 dates between September 1991 and August 1993 and seedling densities were estimated on 21 dates between September 1991 and July 1993. Total aboveground standing crop ranged from about 110 to 340 g m⁻² and was dominated by three grasses: Bouteloua gracilis, a warm-season, perennial; Pascopyrum smithii, a cool-season, perennial; and B. japonicus, a cool-season, annual grass. Seed production was dominated by B. japonicus (∼10,400 seeds m⁻²). Bouteloua gracilis and P. smithii seed production was negligible with maximum densities of about 120 and 25 seeds m⁻², respectively. Density of seedlings was greatest for B. japonicus (∼2100 seedlings m⁻²), Festuca octoflora (∼1100 seedlings m⁻²), and Plantago patagonica (∼350 seedlings m⁻²). Maximum number of B. gracilis seedlings was 25 m⁻². No P. smithii seedlings were observed during the study. Temporal differences in seedling densities were closely tied to late summer-early fall precipitation patterns. Although community-level seed production and seedling establishment processes were dominated by B. japonicus, the overwhelming presence of herbage produced by the two vegetative propagating species, B. gracilis and P. smithii, casts doubt on a trend in ecological succession toward an annual grass-dominated community.