This study documents the causes of mortality of juveniles in a declining population of thirteen-lined ground squirrels (Ictidomys tridecemlineatus). We found a dramatic decline in juvenile numbers between emergence from the natal burrow and the onset of hibernation. From 2005 to 2007, an average 70.6% of all juveniles disappeared from the population (n = 168 males and 224 females), including a 72.0% loss of males and 65.2% loss of females (χ²₁ = 2.1, P > 0.10). Weekly loss of juvenile males peaked 3–4 w after emergence while female loss remained relatively stable through summer. Radio-tracking enabled us to eliminate dispersal as a cause of the decline in juvenile numbers. Predation accounted for at least 71.4% of mortalities in radio-collared juveniles (66.7% of males and 76.0% of females; χ²₁ = 0.087, P > 0.75). We found no sex differences in specific behaviors that could predispose one sex to predation, but males were less wary than females in response to a potentially frightening stimulus.