For a given animal, life consists of a series of decisions and compromises made in an effort to maximize fitness. These include decisions about how much time and energy to apportion to foraging, reproduction and associated social activities, and when to abandon these resources in the presence of a predator. The decision of when to flee is critical to the fitness of an individual (Cooper and Whiting 2007). According to escape theory, there is a balance between the cost of fleeing and the chance of being caught (Dill 1974); this tradeoff results in a decision as to how close a predator should be permitted to approach before initiating flight behavior (i.e., flight initiation distance; Ydenberg and Dill 1986, Cooper and Frederick 2007). Furthermore, there is a tradeoff between the antipredator behaviors of crypsis and flight; namely, when an animal initiates flight, it increases its likelihood of being detected by the predator (Martin et al. 2009). Flight initiation distance (FID) is a trait that varies between populations and can be affected by factors including vegetation cover, type of predator, and predator density and efficiency (Blazquez et al. 1997, Diego-Rasilla 2003, Camp et al. 2012). Flight initiation distance has been studied in lizards extensively (e.g., Cooper and Whiting 2007, Martin et al. 2009, Cooper 2010, Cooper 2011). Generally, in populations exposed to higher predator densities, lizards are more wary and display longer FIDs (Diego-Rasilla 2003); that is to say, they initiate flight behavior while the predator is farther away, as compared to those in environments with lower predator densities. In many animals, predation response is specialized (e.g., Walther 1969, Ghalambor and Martin 2000). The Western fence lizard (Sceloporus occidentalis) can distinguish between potential predators using visual cues about size and movement patterns (Fitch 1940, Fine 1999). This small, primarily insectivorous lizard is ubiquitous in open, sunny habitats west of the Rocky Mountains (Stebbins 2003). Common predators of this widespread species include snakes (e.g., racers, kingsnakes; Fitch 1940), mammals (e.g., foxes, raccoons, shrews; Nussbaum et al. 1983), and birds (e.g., kestrels, shrikes; Fitch 1940, Cooper and Whiting 2007). While encounters with humans may result in the occasional removal of an individual from a population (e.g., collection for the pet trade or incidental death), humans do not generally play the role of predator for Western fence lizards. Consequently, at sites with regular human activity these lizards should not view humans as dangerous predators and FID in response to humans should be reduced. Since flight for FID studies is usually initiated by a human researcher, it would be useful to determine if lizard FID responses differ between populations exposed to different human densities. This information would determine if it is necessary to choose study sites with similar human activity levels in order to remove unnecessary site bias from a study. We measured the flight initiation distances of lizards from two populations of Western fence lizards with different human exposure to determine if there was a difference in FID between the Bull. Southern California Acad. Sci. 113(1), 2014, pp. 42–46 E Southern California Academy of Sciences, 2014