Thurber and Peterson (1991, Journal of Mammalogy, 72:750–755) analyzed data on body mass of North American coyotes, and speculated that the larger size of eastern coyotes was most likely attributed to a phemotypic response to enhanced food supply. We argue that data on food habits, cranial morphometrics, body mass in captivity, and behavior suggest rather that this larger size of eastern coyotes is due to genetic factors and represents an adaptation to a larger prey, namely, white-tailed deer.