Ca 2þ . Because major components of the spasmoneme, the contractile organelle inside the stalk, are EF-hand Ca 2þ -binding proteins including spasmin and centrin, the spasmonemal contraction is thought to be related to other centrin-based motility mechanisms. This study describes how stall force affects contractions of live Vorticella. To impede contractions, we applied hydrodynamic drag force to Vorticella in a microfluidic channel with Poiseuille flow of viscous PVP solution. This method enables controlling the stall force by changing flow rate and the viscosity of the solution. Cell dimension measurements show that the zooid is elongated by the flow in relaxed and contracted states keeping roughly constant volume. As the stall force increases, the end-to-end length of the contracted stalk increases while that of the relaxed stalk is almost constant, and maximum contraction speed decreases while contractions take longer time. Furthermore, the time lag in contraction commencement between the zooid and the stalk also increases. We measured time differences in movement start among polystyrene beads attached to the stalk, and they increase with increasing stall force. These increasing time lags imply that the stalk cannot contract until it develops force great enough to overcome the stall force. The stall force affects the relaxation of Vorticella because relaxations take longer time as the stall force increase and the extending stalk resumes its contraction after the stall force is removed. It seems that although the spasmoneme retains contractile force, the stall force extends the stalk.