Occurrence and ef fects of harmful bloom caused by a kind of redt ide algae , Prorocent rum micans , in seawater experimental enclosure were studied .When high conce ntra tion of re act ive phosphate was available and motion of water was weaken in enclosures , phytoplankton was easily dominated by P .micans , and then se rious h armful bloom of the species formed .The color of water in the enclosures with harmful bloom was br own re d at the beginning and turned to black br own at the post stage .Biomass of phytoplankton , chlorophyll α, gross primary production , respirat ion of water column , chemical oxygen demand , and part icle organic carbon were much higher in the enclosure wi th harmf ul bloom than those without bloom.Both dissolved oxyge n and Secchi depth usually varied dr ama tically with diel vert ical motion of P .micans .Ther e were no significant effects of P .micans bloom on survival and growth of Penaeus chinensis and a hybrid tilapia of Oreochromis mossambicus ×O .ni lot icus to be detected although the ir body color became br own red , but mortality r ate of bay scallops stocked in enclosures with serious harmful bloom was 100 %.Clearly P .micans bloom was more harmful to marine bivalves than to f inf ish and pr awn .