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Environmental factors affecting growth of the threatened eastern sand darter ( Ammocrypta pellucida (Putnam, 1863)) were examined using specimens sampled from the northern edge of its range to determine the species’ critical habitat. Length-at-age increments were determined from scale samples as surrogates for growth rates based on back-calculated lengths using the Fraser–Lee method. During the first year of life, 82% of total length is attained, suggesting considerable energetic partitioning towards reproduction following age-0. Positive relationships between age-0 length increments and sand substrates and between age-0 length increments and mean annual channel discharge indicated greatest first-year growth within sand-dominated, high-discharge habitats. Environmental factors that occurred at coarse spatial and temporal levels (i.e., mean annual channel discharge) explained more of the growth variability among eastern sand darters than those occurring at fine levels (i.e., site-level substrate composition). This study indicates that environmental factors can be used to explain variability in cohort-structured population and site-level growth of eastern sand darters.