Flowering individuals of dwarf ginseng may be either male or hermaphroditic. I recorded the sex expression and size of individuals in three populations for three or four years in order to 1) determine whether this bimodal distribution of sex expression was due to sex changing or genetic dimorphism, and 2) test predictions about a) the relationship between size and gender, and b) the association of size change and sex change. Twenty five to 37% of the flowering individuals in each population changed gender from one year to the next. Of the plants I followed for four years, 83% changed sex and 57% changed more than once. In each of these populations as well as two others, hermaphrodites were significantly larger than males. Gender dynamics of the three populations differed, but hermaphrodites tended to become smaller and were more likely to change gender than remain hermaphroditic the following year, whereas males tended to grow larger and were more likely to remain male than to change gender. Dwarf ginseng is clearly a diphasic (sex changing) species in which sex expression is determined primarily by size. A difference between genders in the immediate resource costs of reproduction appears to be an important determinant of sex change and gender phase ratios in populations.