The physical movements of the swimming 4th-instar larva of Culicoides variipennis were analyzed in relation to temperature, viscosity, and larval age. Additional observations describe changes in behavior with rising viscosity and behavior at a low/high viscosity interface. Culicoides variipennis larvae swim by rapid sinuous flexions of the body (rapid flexion). Analysis of sequential film images showed that deviations from the mean path at 9 equidistant positions along the larva were approximately symmetrical about the midpoint of the body, implying a constant bend angle between body segments during propagation of the wave. The rate of wave propagation (beat rate) was strictly proportional to temperature, increasing from 3.3/s at 6 °C to 18.3/s at 36 °C. Swimming speed also increased with temperature, but the rate of increase declined as temperature increased; this is associated with the fact that the amplitude of the propagated wave diminished with rising temperature. The effects of viscosity were somewhat more complex. Exponential decay equations were used to describe the relationship of both beat rate and wave amplitude to rising viscosity. Swimming speed, since it is dependent on both of these variables, was sharply curtailed as viscosity increased, falling from 16.6 mm/s at 1 centipoise (in water) to 0.7 mm/s at 50 centipoises. Thus, rapid flexion rapidly became ineffective as a means of propulsion as viscosity increased. In tests involving larvae of different ages, beat rate declined logarithmically with increasing larval length, but swimming speed increased according to a power regression model, indicating that larger larvae swim more efficiently relative to their size than do smaller ones. When larvae were placed in 11 methyl cellulose solutions of progressively increasing viscosity, they engaged in 4 behaviors (rapid flexing, slow flexing, looping, and no movement) for varying proportions of the total time. These behaviors are described, and the behavioral changes induced by rising viscosity are illustrated and discussed.