Continuous and well‐marked changes in composition are demonstrated during the development of the cotton boll, and discussed in relation to changes in susceptibility to pests and diseases occurring over this period.Cotton buds and flowers, the main food of the earlier instars ofHeliothisandDiparopsis, and, at the beginning of the season, ofPlatyedra, represent the richest recorded source of nitrogen available to the larvae.The first 2 weeks of boll development, when most physiological shedding occurs, are characterized by extremely rapid growth, the dry weight of the ovules being approximately doubled every 2 days throughout this period.The developing ovules are richest in reducing sugars during the second and third weeks, when moisture‐content is also highest. Bolls of this age are attacked by the later instars ofHeliothisandDiparopsis, which are the stages at which the accumulation of larval fat is likely to be most active. MaleDysdercus, which can survive for prolonged periods on simple sugar solutions, show a marked preference for bolls of this age, which are punctured to a varying extent by other stages ofDysdercus, probably primarily as a source of water. This is also the stage at which attack byNematosporais most damaging; reducing sugars provide a highly suitable source of carbon for this fungus.The cellulose of the mature lint and the oil and protein of the ripe seed are mainly laid down after the boll reaches full size (at 4–5 weeks under local conditions), about half‐way through its maturation period, and are largely derived from materials entering the boll in the course of its subsequent development. Premature senescence, such as that associated withAlternariaattack, can thus affect yield even when defoliation does not occur until after most of the bolls have attained full size.The ripening seed, becoming steadily richer in oil and protein during the latter part of boll development, forms the main food ofPlatyedratowards the end of the season, a change of diet reported to induce the larval diapause. Ripe seed provides food which is essential for satisfactory nymphal development and probably also for oviposition inDysdercus; specific protein requirements are possibly involved.Examples of direct effects of environmental factors on the development and composition of the boll are described. Over the range of conditions experienced by the experimental material these effects were relativel