ABSTRACT The mechanism of retraction of the longitudinal flagellum of Ceratium tripos was studied by making extracted models of the flagellum. Non-detergent models extracted in low ionic strength medium containing 1 M-glucose, 10mM-EDTA and 50mM-Tris. HCl buffer (pH8·0), retracted when Ca2+, Mg2+, Ba, Sr2+, Mn2+ or Cd2+ was applied locally with a glass capillary. A demembranated model of the flagellum was made with an extraction medium containing 0·8-TO M-glucoβe, 20mM-Tris-acetate (pH7·8), 2mM-EGTA, 5–7mM-MgS04, 0·l M-potassium glutamate and 0·l% Triton X-100. The model required a concentration of Mg2+ of a few mmol/1 for successful reactivation of both retraction and undulation, and about 0·l M-potassium glutamate (or sodium glutamate) for reactivation of undulation. Neither type of motion of the models could be reactivated above 35 °C. Ca2+ induced the retraction at pCa 5·5 or less. In addition to Ca2+, Mn2+, Ba2+, Sr2+ and Cd2+ also induced retraction but Mg2+, La3+ or Tb3+ did not. Although ATP was required for undulation, it was not required for retraction. Co-incubation with hexokinase to remove contaminating ATP did not suppress the retraction. The potent ATPase inhibitor, orthovanadate, inhibited undulation at 10μM but did not inhibit retraction even at 2mM. SH blockers, N- ethylmaleimide and dithio-bis-nitrobenzoic acid strongly suppressed undulation but had no effect on retraction. Calmodulin inhibitors, trifluoperazine and chlorpromazine, also had no effect on retraction. These data indicate that undulation is generated by a 9+2 microtubular axoneme using energy released by hydrolysis of ATP and that retraction can be induced by Ca2+ without a requirement for ATP.