The bluemask darter Etheostoma (Doration) sp. is an endangered fish endemic to the upper Caney Fork system in the Cumberland River drainage in central Tennessee. Darters (Etheostoma spp.) are typically short-lived and exhibit rapid growth that quickly decreases with age. Consequently, estimating age of darters from length-frequency distributions can be difficult and subjective. We used a nonparametric kernel density estimator to reduce subjectivity in estimating ages of bluemask darters. Data were collected from a total of 2926 bluemask darters from the Collins River throughout three growing seasons. Additionally, data were collected from 842 bluemask darters from the Rocky River during one growing season. Analysis of length-frequencies indicated the presence of four age classes in both rivers. In each river, the majority of the population was comprised of fish <3 y old. Some age-one individuals were sexually mature, but the majority of the reproductive population was comprised of older fish. Differences in age-class structure among years reflected variable year-class strength in the Collins River that seemed related to summer discharge. Male and female growth rates differed significantly (P < 0.05); males were typically larger than females of the same age after the first growing season and the majority of age-three individuals were males. Growth rates in the Collins and Rocky Rivers were similar for each sex (P > 0.05). In both rivers, females were more abundant than males.