The mechanism of fertilization in Macrobrachium rosenbergii was analysed by cytological observations of the whole process of fertilization which was investigated with artificial insemination (in vitro). Unlike most other arthropods, the Macrobrachium rosenbergii egg has no micropyles. The continuous vesicles at the tip of tubular bundles in the sperm base contain glycoconjugates which may function in the recognition and binding to the egg. In the course of fertilization, several sperms attach themselves to the egg with the lateral edge of the base, and then they bend their spikes to make the tips touch and penetrate into the egg. Only one of them can be completely incorporated into the egg with the spike entering first and the base following. The experiments of fertilization and location of acid phosphatase suggest that the spike of the sperm may play the role of acrosome.