Micrographs were made of free-swimming hamster spermatozoa using a high intensity xenon flash and two-colour darkground illumination - conditions that permit three-dimensional reconstruction of the instantaneous shape of the flagellum. From the waveforms observed, we constructed an account of the three-dimensional kinematics of this flagellum. We found that near-planar bends grew on the proximal 25% of the flagellum to reach a mean angle of 1 X 7 rad. Principal bends achieved a greater angle than reverse bends. As bends propagated they maintained their near-planarity and their angle, but decreased in radius of curvature. However, the plane of the more distal bends tended to become displaced, as though a predominantly clockwise torsion of the axoneme was developing in the interbend segment. This gave rise to a complex shape resembling a sinistral helix of reducing pitch and eccentricity. There are clear indications that not all cycles of bending lead to the same degree of three-dimensionality.

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