Helices are expected to scatter circularly polarized light preferentially. According to theory, the scattering depends on the pitch, radius and sense of helix1-4; however, experimental data have been lacking. We now report quantitative measurements of the differential scattering of left and right circularly polarized light from the sperm head of the octopus Eledone cirrhosa. This sperm head is a left-handed helix with pitch and diameter of about 1 μm. It preferentially scatters left circularly polarized light versus right circularly polarized light in the forward direction, with a maximum differential scattering of 2% at 35° to the incident beam (wavelength = 0.442 μm). The preferential forward scattering of left circularly polarized light is consistent with theory1-4. As a differential scattering of 0.1% is easily measured, this can be a useful new method to characterize helical biological objects. © 1982 Nature Publishing Group.