The effect of a PSP producing dinoflagellate Alexandrium tamarense on early development of scallops (Chlams farreri , Argopecten irradians) has been studied. The results showed:1. A. tamarense could inhibit egg hatching, 36hEC50 was about 1,000cells/ml. The most sensitive stage of egg development to the toxic algae is before archenteron phase and the strongest impact was found when the toxic algae was at its early exponential stage. A conclusion was drawn from our experiments that the adverse effect to egg hatching was caused by some substances, non PSP origin, at the surface of cell membrane of A. tamarense after comparing the effect by other algae, Phaeodactylum tricornutum and Chattonella marina, as well as different fractions of A. tamarense cultures including cell contents, cell fragments, cell free medium and standard STX toxins. 2. Survival rate of scallop larva at early D-shape stage decreased significantly after exposed to A. tamarense for 6 days and its swimming ability could be inhibited in 2 hours, both at concentrations 5,000 cells/ml and above. 3. No obvious effect of A. tamarense was found on metamorphosis, mortality of eyespot larvae during 14-day experiment, but the growth of juvenile scallop was inhibited, with 14dEC50=5,000cells/ml. 4. Climbing ability of juveniles could be clearly reduces by A. tamarense in very short exposure, with 1hEC50=1,000cells/ml. The adverse effect of A. tamarense on early development of scallops especially on egg hatching indicated that A. tamarense bloom may cause decline of shellfish population and may have further impact on marine ecosystem.