ABSTRACT Equations are derived for the propulsive velocities and power dissipation of undulating hispid flagella. The observed rates of movement of water around the flagella of two chrysomonad flagellates and the translational speed of Ochromonas malhamensis agree closely with values calculated if it is assumed that the flagellar mastigonemes remain perpendicular to the flagellar surface and that these mastigonemes lie in two diametrically opposed rows situated in the plane of the flagellar wave.