1. 1. The effects of selected dosages of colchicine (10 −2 to 10 −13 M) were studied in a red-pigmented, heterotrichous ciliate Blepharisma undulans on the following: (1) division, (2) regeneration of cut Blepharisma, (3) conjugation, (4) survival of whole Blepharisma, regenerating and non-regenerating fragments. 2. 2. Concentrations below 10 −4 M were ineffective on all processes studied. Higher concentrations (10 −2 M and above) were cytolytic to Blepharisma. 3. 3. Reconstruction of the red pigment in bleached forms was unaffected by colchicine. The response of bleached forms to colchicine was the same as unbleached organisms. Cannibal-giants were less affected by colchicine on the processes studied than normal forms. 4. 4. The effect on division indicated that in ciliate protozoa where there is an amitotically dividing nucleus, the macronucleus, the division of the cytoplasm is apparently unaffected by disruption of the mitotically dividing micronuclei. 10 −4 M produced micronuclear mitotic inhibition without affecting cytoplasmic division, 10 −3 M affected both processes. 5. 5. Conjugating whole Blepharisma or conjugant fragments were more affected by lower concentrations (10 −4 M) than non-conjugating forms. In some instances conjugating organisms exposed during conjugation did not recover from colchicine exposure. The inability to recover may possibly be due to a disruption of the new nucleus. 6. 6. The process of regeneration in Blepharisma was more affected by lower concentrations (10 −4 M) than was division. The effect on regeneration may possibly be due to a disruption of the ciliary fibrillar systems. 7. 7. An explanation is offered for the change from the normal-sized individual to the cannibal-giant. This change may be due to the production of a polyploid individual.