SUMMARY1.Phlyctochytrium chaetiferumhas been found on dead cells and filaments ofHydrodictyon reliculatumandOedogoniumsp. which had been growing in battery jars in the laboratory. It is characterized by oval and somewhat pyriform extramatrical zoösporangia on which occur from 3 to 30 elongated, hyaline, continuous, branched, comparatively stiff but flexible hairs which may occasionally attain a length of 200μThe intramatrical portion of the thallus consist of a globular, spindle-shaped, elongated, or irregular apophysis and a fairly extensive rhizoidal system oriented on its base or sides.2.The zoöspores are hyaline, spherical, 2.5 to 4μin diameter, with a large clear refractive globule in the center and a single posteriorly attached cilium which is approximately four times the diameter of the spore in length.3.The resting spores are extramatrical, hyaline, oval to spherical in shape, with smooth thick walls and one or more large refringent globules in the center. So far no gametic fusion has been observed in relation to their development, nor have any germination stages been found.