Monostome trematodes of the family cyclocoelidae, taken from the American coot, Fulica americana, were described as a new species, Cyclocoelum microcotyleum, by Noble in 1933. These flukes were found in the air-spaces or the coelomes of birds taken from the waterways near Stockton, California. Before this present work various attempts had been made to determine the lite history of this fluke. A number of fresh-water snails, indigenous to the region in which the definitive host was found, were exposed to miracidia of this fluke, but observations or infection were not obtained. In the winter of 1940-41 specimens of the fresh-water snail Physa sp. were exposed to the newly-hatched miracidia of this fluke, and various stages of the larval forms were observed.