We investigated the life history and demographics of Notropis rafinesquei (Cyprinidae) in three streams in north Mississippi, USA, over 2 y. Size at maturity (22.5–29.3 mm SL) and the timing and duration of the reproductive period (Apr.–Aug.) were similar among all three streams in both years. We consistently detected two young-of-the-year cohorts, one strong cohort appearing in early summer and the other of varying strength in fall. In contrast, we found striking variation in most other life history traits including female length, body weight, clutch size, ova diameter and sex ratios, despite the close geographic proximity of these populations. Life history traits of all other species in the Notropis longirostris clade (N. ammophilus, N. longirostris and N. sabinae) are similar to N. rafinesquei, but like N. rafinesquei, vary widely among and within species. The highly plastic nature of reproductive traits in N. rafinesquei and other members of the N. longirostris clade is likely an important adaptation that allows these animals to maintain large populations in the often physically unstable and unpredictable stream environments of the Gulf Coastal Plain, USA.