Comparisons of sonic muscle width, body cavity length, and sonic muscle innervation patterns were made for three species of sciaenids: weakfish, Cynoscion regalis; Atlantic croaker, Micropogonias undulatus; and spot, Leiostomus xanthurus. Body cavity length and sonic muscle width (rostrocaudal extent), both normalized for TL, were significantly different among the three species, increasing from spot to croaker to weakfish. However, the percentage of the body cavity length including sonic muscle was greater in the croaker and weakfish than in the spot. The sonic muscles of sciaenids are innervated by a number of true spinal nerves rather than by paired occipital spinal nerves. The number of sonic nerves increased from spot (4–5 nerves from vertebrae 4–9) to croaker (6–8 from vertebrae 3–10) to weakfish (8–9 nerves from vertebrae 3–12), paralleling the increase in muscle width. Although the number of nerves and their vertebra of origin varied within each species, they were bilaterally symmetrical within an individual. The spinal nerves emerged from two (dorsal and ventral) foramina in croaker and spot and from the sole foramen observed in weakfish vertebrae. Therefore, a longer body cavity supports a wider sonic muscle that is innervated by a greater number of spinal nerves.