Morphology frequently varies with phylogeny, body size, sex, and phenotypic plasticity. However, the relative influence of these variables is unknown for most taxa. Morphological variation of Freshwater Drum Aplodinotus grunniens in the Wabash River, USA was described using geometric morphometrics. A MANCOVA model of shape indicated that morphological variation was primarily influenced by allometry (body size), sex, and river location. Among all individuals, at least 50% of the variability in morphology was a product of body size while sex and river km (collection locale) accounted for 10% and 5% of the overall variability in shape, respectively. However, when mature and immature individuals were analyzed separately, mature individuals demonstrated no morphological signal concurrent with river km, while at least 45% of the variability in immature shape was attributed to river locale. The contributions of allometry, sex, and river gradient on Freshwater Drum morphology suggest that morphological variation is largely a result of a combination of developmental, sexual, and environmental influences.