The cap cells ofAllium cepa, labeled with 3HTdR, are displaced from the meristem and sloughed from the cap; after 72h the whole cap appears renewed. During the cap cell differentiation processes, some nuclear cytopbotometric and ultrastructural aspects were analyzed. Though all the nuclei of the root cap differentiate with the same Dna content per nucleus, histones/Dna ratio shows differences both between and within columella and peripheral cell nuclei. Besides, histones/Dna ratio shows a wider heterogeneity in mature than in undifferentiated cells. The ultrastructural observations are in agreement with histones/Dna data, showing changes in chromatin structure.