1. The vast swarms of swimming protozoans of the genus Folliculina that were found to settle down over the aquatic plants along the shores of side branches of the Chesapeake Bay in 1912 and 1913, came in even greater numbers in 1914, and it is therefore probable that this immigration and colonization is a regular annual phenomenon. 2. The incursions of swimming Folliculina do not take place as soon as the plants have grown enough to supply places for attachment, and the departure or disappearance of the living Folliculinas antedates the cessation of growth and final dying down of the plants upon which they settle. 3. As far as evidence is available the numbers that crowd the leaves arise more from immigration from without the area than from division of animals that have already settled in the area. 4. The times of appearance and disappearance differ in successive years. 5. It is suggested that conditions of food possibilities are determining factors in these inroads into the brackish fauna. 6. The great number of free swimming forms makes them, for the time being, an important factor in the plankton. 7. The crowding of the dwellings or cases on the leaves all along the shores is a considerable element in the transformation of matter which, arising from decay of organic materials, is transformed into bacteria and other plankton organisms, which in turn are eaten by Folliculina and enable them to secrete resisting tubes and sacs which finally settle to the bottom of the river.