The North American porcupine (Erethizon dorsatum) has expanded its range into the Edwards Plateau of Texas, an area harboring a disjunct relict population of papershell pinyon pine (Pinus remota). We captured 39 porcupines and monitored 37 of these animals by radiotelemetry from 1997 to 1999 to assess their demographic characteristics and determine their potential impact in this unique wooded ecosystem. The adult population was biased toward females (3 females: 1 male), and annual rates of survival were greater (P < 0.01) for adult females (S = 0.91, 95% ci = 0.77–1.00) than for adult males (S = 0.42, 95% ci = 0.00–0.85). Birth rates of reproductively mature females averaged 0.82 annually during the 3-y study. We calculated a mean reproductive rate of 0.41 female young/female based on birth rates and juvenile sex ratio. The estimated population growth rate of 0.034 indicated that the study population was growing. Direct and indirect ecological effects of porcupines in this woodland ecosystem warrant investigation.