The objective of this study was to determine dietary overlap between blueback herring and threadfin shad in J. Strom Thrumond Reservoir, South Carolina/Georgia. We also evaluated prey selectivity for each speices and diet differences between two size categories of blueback herring. Diet and zooplankton samples were collected every other month from April 1992 to February 1994. We examined stomachs containing prey from 170 large blueback herring (>140mm), 96 small blueback herring (<140mm), and 109 threadfin shad, and we also examined 45 zooplankton samples. Large blueback herring diets differed significantly from threadfin shad diets on 11 of 12 sampling dates, and small blueback herring diets differed from threadfin shad diets on all sampling dates. In general, blueback herring consumed proportionally more copepods and fewer Bosmina sp. and rotifers than threadfin shad. Large and small blueback herring diets were significantly different on five of eight sampling dates, primarily due to the tendency of small blueback herring to eat proportionally more Bosmina sp. than large blueback herring. Both blueback herring and threadfin shad fed selectively during some periods of the year. Diet differences between the species may contribute to their coexistence; however, both blueback herring and threadfin shad showed a strong preference for Bosmina sp., increasing the chance that they may negatively influence one another.