Abstract: A small flagellate （strain no. MMDL4071） was isolated from the phycosphere of dinoflagellate Alexandrium tamarense. The morphological observation demonstrated that the cells of the flagellate were spherical， measured 3 μm and 2 unequal flagella arisen from the anterior face to the cell. The ultrastructure observation by scanning electron microscope showed that the cells of the flagellate contain nucleus， chloroplast， chloroplast endoplasmic reticulum， Golgi body， mitochondria， and "9+2" microtubule structure flagella. Control experiment proved that the flagellate was heterotrophic species， and the binary fission process was captured. Combine with the morphological， ecological characteristics and phylogenetic analysis， this heterotrophic flagellate was identified as Ochromonas sp．