A model which relates optimal food preference relationships and caloric yield/unit time of potential food sources is derived. It is suggested, on the basis of this model, that: food preferences can be adequately described only if a number of factors other than relative frequencies in the diet and relative abundances of the food types are known; animals should be more selective in their choice of foods when satiated or when food is common, more indiscriminate when starved or when food is scarce; animals may eat 1 food type with greater frequency, relative to its abundance, than another even if the other food is richer and more efficiently exploited. This occurs in situations of high relative abundance of the 1st food type; the extent to which predators tend to pass by potential food items may be used to evaluate the role of food in the population limitation of a predator sp; food preferences appear to change readily and appropriately to changes in the environment.