Current and historical age structure information on the Yellowstone–Sakakawea stock of paddlefishPolyodon spathulais reviewed in relation to the completion of Garrison Dam in 1953 and the subsequent filling of Lake Sakakawea, a 156,000-ha impoundment on the Missouri River. Paddlefish abundance increased greatly after the closure of Garrison Dam, and the first large group of paddlefish (nearly all males) migrated up the Yellowstone River to Intake, Montana, in 1962–1963. By the mid-1970s, after the gradual filling of the reservoir (1954–1966), both females and males had fully recruited to the Intake fishery, and more females were harvested than males. By the 1980s, older females predominated in the harvest. Based on age determination in 1991–1993, the stock has continued to age, from a mean of 10.3 years in 1964–1965 to 14.8 in 1974, 20.3 in 1985, and 23.5 in 1992. In 1993, however, mean ages of paddlefish at Intake decreased by 2 years for males and 3.3 years for females. In 1991 and 1992, males from Intake were characterized by bimodal age distributions with ages ranging from 7 to 40 (means, 18.4 and 17.8, respectively); females demonstrated unimodal age distributions and ranged in age from 10 to 42 (means, 25.5 and 26.1). Males commonly matured at age 9. but females almost never matured before age 15. The oldest paddlefish encountered was a 14-kg male caught in 1985 and estimated to be 55 years old. Although substantial recruitment has occurred within the past decade (as indicated by recruitment of 9–11-year-old males to the fishery in 1991–1993) and reproduction appears to have occurred in 1991 and 1993 (based upon counts of young-of-the-year in Lake Sakakawea), the age structure suggests that recruitment may be less in recent years than in the years soon after the reservoir was closed and filled. A conservative harvest is called for until it is clear that reproduction is adequate to sustain the stock and the fishery.