Abstract: When we studied the allelopathic effects of rice straw on the growth of microalga Chlorella pyrenoidosa, a flagellate strain which could strongly graze on C. pyrenoidosa and effectively inhibit the population growth of C. pyrenoidosa was incidentally isolated. The flagellate had two yellow chloroplasts, cell spherical or oval, sometimes metamorphic, and usually freeswimming but occasionally attaching to surfaces with posterior end. The flagellate had two unequal flagella. The longer flagellum was about 1.5 times as the length of the cell, and the shorter one was about half times as the length of the cell. The flagellate showed strong phagocytic capacity on C. pyrenoidosa. Each flagellate could simultaneously swallow 3-4 cells of C. pyrenoidosa, and its body size changed greatly with the quantity of food swallowed. From the morphological features, this flagellate was identified as a species of Ochromonas. The growth curves of the Ochromonas sp. population on three different culture media (SE, rice straw, and wheat grain) showed that wheat grain medium was optimum
 for the population growth. Based on our results, the trophic strategy of Ochromonas sp. was of mixotrophy, but phaghotrophy was the main mode.

Key words: Chilo suppressalis, Spatial distribution pattern, Geostatistics