Tetrahymena vorax strain V2 type S is a small polymorphic ciliate which transforms from a small saprozoically feeding mlcrostome (small-mouthed form) into a large carnivorous macrostome (large-mouthed form) when microstomes are suspended in distilled water with prey (Tetrahymena pyriformis). The events associated with this type of transformation are as follows: Macrostome formation involves changes in the oral apparatus and results in complete resorptlon of the small mouth and its replacement by a larger mouth, which forms from an anarchic field arising directly behind the resorblng (small) mouth. This process is called oral replacement. Concomltantly with oral replacement, there is formation of a large pouch posterior to the oral apparatus. This pouch serves as a reservoir for captured prey and is termed the pharyngeal pouch. The reverse transformation, that from macrostome to mlcrostome, always involves division of the macrostome to form two mlcrostomes. In this process the oral apparatus of oplsthe (future posterior daughter cell) arises as a small oral anlagen which then differentiates into the oral apparatus of the mlcrostome. The large mouth (macrostome) of the proter (future anterior daughter) is reformed and reshaped until a small mouth (microstome) is formed. By the time the proter and oplsthe separate both cells are mlcrostomes. During this division the pharyngeal pouch disappears.