Inbreeding depression in a population of an endangered autotetraploid plant, Aster kantoensis (Asteraceae), was estimated over a 2 year period. The first-year survival rate of the outcrossed progeny was significantly higher than that of the selfed progeny. After 2 years the number of flowers per reproductive individual was also greater in the outcrossed progeny, and the composite value of inbreeding depression was 0 790. The multilocus estimate of outcrossing rate in the population is 0.883 ± 0 064. The high value of inbreeding depression is consistent with the predominant outcrossing of the population.