The effect of colchicine on stalk elongation in the colonial peritrich ciliate Carchesium sp. has been investigated by growing this protozoon in colchicine-containing media. The length of the stalk in control cultures was 0-4-0-9 mm. In the presence of 2-5-12-5 mM colchicine, stalk elongation was inhibited, and stalk length was inversely proportional to colchicine concentration. At concentrations above 7-5 mM colchicine, stalks measured less than 0-1 mm, and sometimes contained imperfect myonemes. The rate of cell fission was retarded in colchicine-containing media, but nevertheless short-stalked colonies with apparently normal zooids were formed. On transfer of such colonies to media without colchicine normal growth was resumed, but only the newly formed branches were of normal length and contractility. The inhibitory effect of colchicine was annulled by Ca2+ and Mg2+ at 10(-3) and 10(-4) M, respectively. At lower concentrations of Mg2+, but in the presence of Ca2+, the effect of colchicine was less conspicuous than at low Ca2+ concentration in presence of Mg2+. Lowering Mg2+ concentration at low Ca2+ concentration, increased the inhibitory effect of colchicine. It is concluded that colchicine-sensitive, probably tubulin-like proteins, participate in the formation of the contractile stalk of Carchesium. Ca2+ and Mg2+ probably compete with colchicine for a common site in these proteins, or they might reduce the cell's permeability to this drug.