We propose that the predator–prey relationship between California ground squirrels,Spermophilus beecheyi beecheyi , and northern Pacific rattlesnakes, Crotalus viridis oreganus , offers a compelling analogy with the well-studied case of intraspecific fighting and assessment. Because ground squirrels frequently place themselves at risk by harassing rattlesnakes, they stand to benefit from assessment strategies which serve to mediate risk. For example, larger and warmer snakes are more dangerous than smaller and cooler ones. These determinants of dangerousness covary with acoustic characteristics of the rattling sounds elicited by squirrel harassment. To determine whether squirrels use these acoustic cues regarding rattlesnake body size and body temperature, we played back rattling and control sounds to individuals in a population of free-living squirrels. The squirrels clearly associated rattling sounds with rattlesnakes and proved capable of assessing both determinants of rattlesnake dangerousness on the basis of acoustic cues. Several features of squirrel behaviour covaried with these acoustic cues, including tail flagging, bipedal posture, and hesitancy to reapproach the area where the rattle was heard. Many of these behavioural differences were sustained for up to 10min postplayback.