Nucleoplasmin is the most abundant protein in the nucleus of Xenopus laevis oocytes. We cloned a cDNA coding for nucleoplasmin from an expression library of immature Xenopus laevis oocytes. The deduced amino acid sequence shows that the carboxyl terminus is very hydrophilic and contains an unusual stretch of 12 glutamic acid residues, which is consistent with one of the proposed functions of nucleoplasmin--that of promoting chromatin assembly. The last 50 amino acids are lysine- and alanine-rich and contain short stretches of homology to histone H1. These regions could be involved in interactions with nucleosomes. The levels of nucleoplasmin mRNA and protein during oogenesis and embryogenesis were investigated using Northern blots, Western blots, and in situ hybridization to oocyte sections. The mRNA is detected during oogenesis but not during embryogenesis, suggesting that nucleoplasmin may be an exclusively maternally expressed gene. However, the protein is present throughout embryogenesis and undergoes pronounced changes in its level of phosphorylation during maturation of the oocyte and just after midblastula transition. These results support the notion that nucleoplasmin is not only important in oocytes but also plays a major role during the rapid cleavages of early embryogenesis.