Beating cilia are important organelles, not only for water pumping in many active filter-feeding organisms, but also for the swimming activity of ciliates and other aquatic organisms that use cilia for propulsion. The present study concerns the effect of temperature-dependent viscosity of the ambient seawater on the swimming velocity of the ‘jumping’ ciliate Mesodinium rubrum in which the propulsion is due to the active beat of an equatorial ring of swim-cilia. This was done by using videomicroscope recordings of ciliates at different temperatures and, at constant temperature, by addition of a high molecular weight polymer (PVP) to manipulate the viscosity. Both ‘large’ (45 mm long) and ‘small’ (22 mm) M. rubrum were studied in order to characterize the jumping behaviour and swimming mechanism in more details. For large M. rubrum, the swimming velocity decreases with decreasing temperature, hence increasing viscosity, from 9.6 +/- 0.3 mm/s at 21C to 5.2 +/- 0.7 mm/s at 9.8C for seawater, and down to 3.7 +/- 0.5 mm/s at a temperature equivalent Te = 5.8C for PVP-manipulated viscosity, and further, the swimming velocity was found to decrease with increasing viscosity according to the power law Vs ~ µ^-n, n~1.93. For small M. rubrum, swimming velocity decreased from 6.1 +/- 1.3 mm/s at 21.1C to 3.8 +/- 0.3 mm/s at 9.5C, while the power-law exponent was n ~ 1.4 and 3 for changing temperature and temperature equivalent, respectively, but with n ~ 1.96 for all data taken together. The results, supplemented with an analysis of a hydrodynamic model for self-propagation of an idealized micro-organism, support the hypothesis that the response is mainly physical/mechanical rather than biological. Since the jump-speed of M. rubrum is nearly the same for all tracks of varying jumplengths at a given viscosity, this indicates that the swim-cilia may frequently have more than one beat cycle per jump, and possibly at times less than one beat cycle. The jump-length to jump-time for large ciliates is larger (~0.5 mm to 101 ms) than for small ciliates (~0.15 mm to 30 ms). However, swim-velocities - when reaching the near-constant level - show less difference, being about 5 mm/s on the average for the temperature range studied. The beat frequency of swim-cilia in jumping ciliates is estimated to be about 60 Hz, which is high but likely necessary for attaining the high swimming velocities observed.