Frontonia collected from stagnant ponds contained innumerable zoochlorellae, which gradually disappeared when transferred to a medium of fresh spring water. The number of zoochlorellae increased when Frontonia. containing relatively few were placed in a stagnant medium. An explanation of this was sought by varying the osmotic pressure of the medium by adding different amounts of glucose. The Frontonia which lived longest in highly concentrated solutions were those containing many zoochlorellae. The stagnant media, in which Frontonia harbored the largest number of zoochlorellae always showed the highest pH; conversely, the pH value of media in which the Frontonia contained little or no zoochlorellae was either slight or negative. There is thus a definite relation between number of zoochlorellae present in a host and degree of organic concentration and pH value of the medium.