We tested the hypothesis that polarization sensitivity enhances the detection of moving, transparent objects by examining the escape response of the red swamp crayfish (Procambarus clarkii Girard) from a visual threat. A transparent, birefringent target trans-illuminated by either partially linear polarized or unpolarized light was advanced toward individual crayfish. The optical axis of the target was aligned such that it would be conspicuous to a viewer with polarization sensitivity when trans-illuminated by polarized light. Under polarized light, significantly more crayfish retreated from the target than under unpolarized light of identical intensity (P<0.00005, Fisher's exact test). Whereas the potential for polarization sensitivity has been shown in neurophysiological and structural studies of the visual system of P. clarkii and the signal crayfish Pasifastacus leniusculus (Dana), our results provide the first behavioral evidence for polarization sensitivity among crayfish. The ecological function of this ability is unclear, but it may enhance the detection of fish with silvered scales, transparent zooplankton or macroinvertebrates. Because escape responses are generally more reliably induced than other behaviors, the method employed in this study may prove useful for examining sensory capabilities in other species.