AbstractSperm of Hemicentrotus pulcherrimus undergo the acrosome reaction in the jelly coat or on the surface of the vitelline layer of Pseudocentrotus depressus eggs and can fertilize the latter, whereas those of P depressus do not undergo the acrosome reaction even after they strike the vitelline layer of H pulcherrimus eggs and cannot cross‐fertilize.In the latter combination, however, if cross‐insemination is performed in the presence of homologous egg water, cross‐fertilization becomes less difficult than in normal seawater, and percentage cross‐fertilization rises as does percentage acrosome reaction. It is suggested that the cross‐fertilizability of this combination depends highly on the inductivity of the acrosome reaction. The reason why such a causal relation is observed is