Phenology, population dynamics, survivorship, and sexual reproduction were studied in two populations (A and B) of Senecio antennariifolius from 1986 to 1988. Senecio antennariifolius flowered from mid-April to late May; by mid-June, all seeds were dispersed. Number of adults in population A decreased by 15% between 1986 and 1987, and by 49% between 1987 and 1988. However, due to a large number of seedlings in 1988 the total number of genets remained nearly constant over a three-year period. In population B, the number of adults decreased by 17% between 1987 and 1988, but the number of seedlings in 1988 more than replaced the number of adults that died. Adults produced flowers when they had 4 rosettes in population A, and 10 rosettes in population B. Mortality was greatest among small adults, and was caused by erosion, invasion of Lonicera japonica, and possibly failure to produce new basal rosettes.