We postulated an essential role for a cysteine-protease in sea urchins sperm histones degradation which follows fertilization. We now report the purification of this enzyme, the determination of its N-terminal amino acid sequence and the localization of the protein with antibodies generated against this amino-terminal peptide. The immunofluorescence data confirmed the presence of this enzyme in the nucleus of unfertilized eggs. After fertilization labeling is observed both in female and male pronuclei suggesting a rapid recruitment of the enzyme to the male pronuclei. Interestingly, we have found that this cysteine-protease persists in the nucleus of the zygotes during S phase of the cell cycle and co-localizes with α-tubulin that organizes the mitotic spindle during the initial embryonic cell division. © 2004 Wiley-Liss, Inc.