Using an equation system based upon the requirements of searching for prey, and the physiological properties of the predator, the hypothesis that growth efficiency of pelagic predators may be importantly determined by the metabolic cost of searching for prey is examined. The equation system indicates that growth efficiency decreases when the prey organisms become either rare or small in relation to the size of the predator. The system also indicates that sustained growth requires increasingly large prey organisms even if these are increasingly rare. Review of empirical literature indicates that the system's predictions are in accord with observation.