When dense (accessory) fibres are released from rat, hamster and rabbit spermatozoa by ultrasonic treatment, their fragments spontaneously adopt helical forms. The pitch of the helices is positively correlated with fibre thickness. The chirality is always sinistral. In becoming helical the fibres must undergo torsion, since the abaxial surface of the fibre always lies on the interior (concave) edge of the helix. A collapse or shortening of the cortical structure of the fibre would have this result. The helical shape cannot be explained by the bending behaviour of attached microtubular doublets, nor can it be merely a response peculiar to sonication. Sperm dense fibres develop as appendages to axonemal doublet microtubules and it is of interest that such doublets, when isolated from sea urchin spermatozoa, also form left-handed helices [23]. Either the axonemal complex is maintained under tension in vivo or the tendency to coiling is suppressed by readily dissociated components of the dense fibres.