ABSTRACT.Drugs that interact with microtubules (colchicine and vinblastine) and microfilaments (cytochalasin B) partially inhibited cell growth and motility ofTritrichomonas foetus.Parasites incubated with these substances became rounded and cell division was blocked. Neither colchicine nor vinblastine disrupted the microtubules that form the peltar‐axostylar system. Any one of these drugs interfered with the net negative surface charge ofT. foetusas evaluated by determination of the cellular electrophoretic mobility (EPM). The decrease in the EPM of cytochalasin B‐treated cells was caused by dimethylsulfoxide, which was used as solvent. Untreated cells as well as cytochalasin B‐treated cells showed a uniform distribution of anionic sites on the plasma membrane as seen with cationized ferritin particles. In cells treated with colchicine or vinblastine the anionic sites were distributed in patches. These results are discussed in terms of participation of labile cytoplasmic microtubules and microfilaments in the control of the distribution of anionic sitecontaining macromolecules located on the cell surface ofT. f