We have previously reported that two trypsin-like enzymes, acrosin and spermosin, play key roles in sperm penetration through the vitelline coat of the ascidian (Urochordata) Halocynthia roretzi[Sawadaet al. (1984),J. Biol. , 2900±2904; Sawadaet al. (1984), Dev. , 246±249]. Here, we show the amino-acid sequence of the ascidian preprospermosin, which is deduced from the nucleotide sequence of the isolated cDNA clone.