The whole sperm of sea urchins, Anthocidaris crassispin and Hemicentrotus pulcherrimus, were found to contain neuraminidase and 4-methylumbelliferyl α-mannosidase. The maximal activity was attained after a lag of a few minutes. The sperm hydrolyzed 8 kinds of 4-methylumbelliferyl glycosides including hexosides, hexosaminides and a fucoside. The enzymes were found to be membrane-bound and solubilized by KCl, sodium taurocholate or Triton X-100. When the enzymes were solubilized, the pH optima were at about 5.5, although the optima were at about 7.5 in the whole sperm. Divalent cation requirement was not detectable. On gel filtration, neuraminidase was eluted as a low molecular weight form, whereas α-mannosidase was eluted near the void volume as a high molecular weight form. The activity hydrolyzing other 4-methylumbelliferyl glycosides was found to be concentrated in the fraction showing α-mannosidase activity. Our results suggest that when the sperm are mixed with egg water, glycosidases on the sperm is fixed at sugar residues in the heterosaccharide chains of the jelly network and hydrolyze the glycosidic bonding after a lag period. The sequence of reactions is supposedly responsible for the reversible agglutination of the sperm by egg water.