Pollen―a substance produced by flowers of plants and collected by bees―is the chief source of protein food of these insects. A lack of pollen causes a cessation or a slackening of brood-rearing, while pollen coming to the hive acts as a stimulus to brood-rearing. When there is a plentiful supply of pollen in the hive and other conditions are favorable, brood-rearing progresses rapidly. During the period 6f mass feeding (the first two days of larval life), royal jelly, a concentrated food of a predigested nature secreted by the younger worker bees, is supplied abundantly. For the remainder of larval life, undigested pollen is mixed with the food derived from a secretion. The bee, when eating pollen, scrapes it from the stored mass or from the pellets brought to the hive by worker bees. The pollen grains are worked between the mandibles by an alternate up and down motion, and then they are gradually worked up to the mouth. The mandibles, or jaws, have a ridge on the concave inside surface which is armed with short, strong spines, and the mandibles probably function as crushing or cracking organs in the preparation of pollen for ingestion. Pollen tubes, as in germinating pollen, have not been seen in pollen within the digestive organs of the bee, but grains which are cracked, or with a portion of the wall missing, are often found. The coats of pollen grains are made up largely of celluloses which the bees are unable to digest. The contents of pollen grains in the normal expanded state completely fill the shell, but after passing through the alimentary canal of the bee, the contents shril1k in many cases, due to the digestion of a portion of the mass. This occurs equally in the adult and in the larva. Some of the simple sugars and proteins are available to the bee, and these are the only food materials which occur in pollen that it is able to use as far as is known at the present time. Starches, dextrins, celluloses, pentosans, some of the simple and higher sugars, and the majority of the carbohydrates are not available to the bee, and probably this is also true of fats and oils which occur in pollens. Proteolytic enzymes―minute chemical bodies which cause complex proteins to break down into simple proteins, thus making these products available as food―have been shown to be present in the alimentary canal of the bee, and it is evident that the bee is able to use certain proteins found in pollen, but as yet these have not been identified. Substitutes for pollen, such as finely ground rye, wheat, oats, pea meal, com, buckwheat, and Mellin's food are not found to be beneficial. Larvae fed these foods die just after the period of mass feeding. During this early period, feeding is not as abundant as when pollen is present in the diet of the bee. The feeding of substitutes stimulates the queen to lay eggs and the nurse bees to feed the young larvae, but development is not completed. If the larval stage could be completed on a substitute for pollen, it is probable that the larvae could not transform to pupae, due to weaknesses, as is indicated by the fate of the queen larvae which completed their larval growth and then died before the change to pupae occurred. The substitutes used in this work do not take the place of pollen in the diet of the bee, and beekeepers in regions which at times experience a shortage of pollen during the early part of the active season should preserve combs of pollen for use at such times, since they cannot depend on the use of substitutes here studied.