The proportion of ortho-gonally orientated responses in a population of daphnids is a function of the logarithm of the intensity ratios of the two orthogonal eliciting stimuli. Use of an overhead beam of plane polarized light with a pair of horizontally opposed, variable intensity, non-polarized light beams propagated parallel to the polarization plane of the overhead beam gives a proportion of orthogonally oriented responses consistent with the theoretical predictions of the Brewster- Fresnel internal reflection model of polarization plane detection.