New specimens of the kleptoplastidal dinoflagellate, Gymnodinium eucyaneum Hu were collected from China.Systematics of the dinoflagellate and the origin of its symbiont were investigated through light morphology and phylogengetic analyses based on multiple DNA sequences.Cells were dorsiventrally flattened.The sharply acute hypocone is conspicuously larger than the hemispherical epicone.G.eucyaneum and G.acidotum were very similar morphologically, and the main difference between the two was the relative size of the hypocone and epicone.