Surface ultrastructure of the flagellates was observed by scanning electron microscopy study of individuals retained on the tissues of the cladocerans. Tissues were fixed with 2.5% (v/v) glutaraldehyde and 2% (w/v) paraformaldehyde in 100 mM cacodylate buffer (pH 7.4) for 30 min at room temperature. After fixation, samples were rinsed three times for 10 min with 100 mM cacodylate buffer, and dehydrated through a graded ethanol series. After washing three times with hexamethyldisilazane (Electron Microscopy Sciences), flagellates were coated with gold and analyzed on a LEO 1430 scanning electron microscope. Morphometrics were determined from examination of SEM specimens.