Abstract: Symbiodinium species live predominantly in mutualistic symbiosis with various invertebrate and protist hosts, and they commonly exist in subtropical and tropical marine ecosystems. Four strains of S. voratum were isolated from the coastal waters of China and from a cargo ship ballast water. Motile cells of S. voratum are small (length 9.7 ± 1.3 μm; width 8.7 ± 0.9 μm), and the immotile cells (diameter 11.4 ± 0.3 μm) and doublet cells (diameter 12.2 ± 0.6 μm) are often packed with storage products. Scanning electron microscope observations reveal the presence of a single elongated apical vesicle (EAV), which exhibits the Kofoidean plate tabulation: x, EAV, 4′-5′, 5a-6a, 9′′, ?s, ?c, 6′′′, 2′′′′. Transmission electron microscope examination shows the presence of a type E eyespot and singlestalked pyrenoid. The golgi is composed of many dictyosomes and the chloroplast thylakoids are in stacks of three. Four S. voratum strains share identical SSU rDNA, ITS, LSU rDNA, cp23S and cob sequences. Molecular phylogeny based on LSU rDNA sequences using maximum likelihood and Bayesian inference revealed that S. voratum from China and other parts of world formed a well resolved clade E. The high genetic similarity among Chinese strains and those from elsewhere suggest frequent gene exchange among them.