Abstract: 13 surveys on causal plankton of red tides were made from April to October 19T9in Dalian iTan. It revealed that the biomass of SkeLetanema castatem (Greville) Clovereached a coxicentration more than10⁴ cells/rnl in 5 surveys, while the hiornass of DactyliosoLera mediterraa Peragallo, Mesodisziwm rurxcm (Lohnzann) and Prorocen-tram micrcas Ehernberg reached more than 103 cells/ml in few surveys and morethan ten. species of common red tide causal organisms were also observed.