Abstract: The effect of cytochalasin B on cortical cytoskeletal proteins of the complementary mating types of Tetrahymena thermophila BF1 and BF5 during their conjugation was carried out, especially during the initial stage of fusion membrane formation stage and the stage of pronuclei exchange. We found that after the treatment of CB, 146KD cortical cytoskeletal protein disappeared, while it existed in the control group, and the percent of 27KD、43KD、47KD and 174KD declined, but the percent of 32KD、41KD、51KD and 54KD kept almost unchanged. Thus we concluded that 27KD、43KD、47KD、146KD and 174KD may be the microfilament proteins.