Abstract Two patterns of ConA binding to starved mating types of Tetrahymena pyriformis were observed depending on its time of addition. When ConA was added upon mixing of the mating types, at zero time of conjugation, it was first bound to the oral region and subsequently was taken into intracellular vacuoles. When it was added to conjugants, it was specifically bound as a ring around the conjugation area. The ability of the cells to form vacuoles, assayed by addition of carmine particles, declined prior to pair formation. The relationship between the above phenomena and the ability of ConA to inhibit conjugation is discussed.