Abstract Analysis of acid-soluble proteins extracted from nuclei isolated from eggs and embryos at various developmental stages during early embryogenesis of the sea urchin Strongylocentrotus purpuratus indicates that arginine-rich histones are the first histone fraction to appear in the nucleus. It has been possible to time their appearance as occurring between the 4 to 8 cell stage and the early blastula. During the first few cell divisions after fertilization, fractionation of the acid-soluble nuclear proteins on Amberlite CG-50 columns using a gradient of guanidinium hydrochloride for elution revealed that no 14C-labeled amino acid entered the biosynthesis of histones. Typical lysine-rich histones appear in the nucleus only later, during hatching and gastrulation. At hatching and later stages, but not earlier, 5% perchloric acid extracted proteins containing 25% lysine could be detected. The amino acid composition of such fractions closely resembled the very lysine-rich histone fraction f 1 derived from calf thymus nuclei.