In this paper, the effects of Alexandrium affine on the survival, growth and reproduction of Mysid Neomysis awatschensis were studied. The results showed that A. affine had an acute lethal effect on N. awatschensis and the 96 h LC50 was about 5 000 cells·mL-1. The toxicity comparison of differen t fractions (Algal culture, resuspended cells, freecell medium, and cell con tent) showed that the algal culture and resuspended algal cells had the remark able adverse effect on the survival of N. awatschensis, while th e freecell medium and cell content also had the adverse impact, although not s ignificant, indicating that the toxicity of A. affine was mostly related with the living algal cells. Chronic experiment (63 days) showed that t he reproduction of N. awatschensis was obviously affected at 50 cells·mL-1 A. affine. At the end of the experiment, the h ighest number of juveniles per day, the total numbers of juveniles produced and the total releasing days were reduced, and the first releasing time was delayed ， as well as reproduction break of the parent mysids were increased， when expo sed to the algae. It was also found that A. affine could affect the survival and growth of the parent mysid: in 50 cells·mL-1 A. affine, the survival rate of N. awatschensis was 71%. The length and weight of adult shrimps were 87.3% and 97.8%, respectively, but the difference was not significant（P＞0.05）. Therefore, A.affine could have adverse influence on population and resources of crustacean.