Sperm proteasomes are thought to be involved in sperm binding to and in sperm penetration through the vitelline coat of the eggs of the stolidobranch ascidian Halocynthia roretzi. However, it is not known whether they are involved in the fertilization of eggs of other ascidians. Therefore, we investigated whether sperm proteasomes are also involved in the fertilization of the eggs of the primitive phlebobranch ascidian Ciona intestinalis. Fertilization of the eggs of C. intestinalis was potently inhibited by the proteasome inhibitors MG115 and MG132 but not by the cysteine protease inhibitor E-64-d. On the other hand, neither fertilization of the vitelline coat-free eggs nor sperm binding to the vitelline coat was inhibited by the two proteasome inhibitors at a concentration sufficient to inhibit fertilization of intact eggs. These results indicate that the proteasome plays an essential role in sperm penetration through the vitelline coat rather than in sperm binding to the coat or in sperm-egg membrane fusion. The proteasome activity, which was detected in the sperm extract using Suc-Leu-Leu-Val-Tyr-MCA as a substrate, was strongly inhibited by both MG115 and MG132, and was weakly inhibited by chymostatin, whereas neither leupeptin nor E-64-d inhibited the activity. The molecular mass of the enzyme was estimated to be 600-kDa by Superose 12 gel filtration, and the activity in sperm extract was immunoprecipitated with an anti-proteasome antibody. These results indicate that the proteasome present in sperm of C. intestinalis is involved in fertilization, especially in the process of sperm penetration through the vitelline coat, probably functioning as a lysin.