The effective population size (N-e) is formulated based on a stage-structured population model and is estimated for two populations of Fritillaria camtschatcensis (L.) Ker-Gawl. (Liliaceae), a perennial, mainly clonally reproducing herb. Plants in these populations change life-history stages year by year, either upward or downward across three unambiguously identifiable stages: one-leaf, nonflowering; multileaf nonflowering; and multileaf, flowering stages. Plants of all stages produce clonal progeny (bulblets) each year, and death of plants occurs only in the first stage. The populations are nearly at equilibrium in both population size and stage structure. N-e is estimated to be 20-30% of the census population size (N), leading to the prediction that a population size of about 20,000 or more will be needed to conserve the normal level of the gene diversity (N-e greater than or equal to 5000). With the current demographic pattern of this species, accelerated growth of the first-stage plants with reduced survival of the second- and third-stage plants will increase both the annual (N-y/N) and generation time (N-e/N) effective sizes of population.