Changes in fine structure and starch, nitrogen, and soluble sugar content were followed through to maturation in developing cotyledons of Vicia faba. Various ultrastructural changes were observed in the developing cotyledons, notably an increase in the number of membrane-bound ribosomes which corresponded with the onset of storage protein deposition. The build-up of storage protein was shown to occur in the cytoplasm within membrane-bound vacuoles which subsequently became the protein bodies of the mature seed, retaining the original tonoplast as the bounding membrane of the protein body. Nuclei became lobed during the later phases of maturation; phytoferritin was observed in plastids of mature seeds. The deposition of reserves in the cotyledons was complete by 85–90 days after flowering, following which water was lost until the seed became hard and ‘ripe’ by no days after flowering.