During 1991–1993 we radio-collared and monitored 50 porcupines (Erethizon dorsatum) on two study sites in central Massachusetts to quantify habitat use. During summer, porcupines were found resting mostly on tree limbs (≥ 80% of locations) in daylight hours. During winter, they rested mostly in dens (e.g., rock caves, cavities in trees) (≥70%) in daylight hours. Where talus was abundant, porcupines rested in dens consisting of rock caves (90%). Where talus was absent, they rested in cavities in trees (50%) more than rock caves (27%). During summer, their use of oaks (Quercus spp.) (60 and 50% of daytime locations in each of two study areas) for feeding and resting was more than expected, based on the abundance of oaks in the two areas (corresponding abundance = 8 and 18%, respectively). During winter, their use of hemlocks (Tsuga canadensis) for feeding and resting when not in dens (93 and 71% of daytime and nighttime locations, in each area, respectively) was more than expected (6% in each area). The diameter at breast height (dbh) of red oaks (Quercus rubra) (x̄ = 52 cm) used by porcupines was larger than a random sample in the area (x̄ = 27 cm). The dbh of hemlocks used for resting and feeding (x̄ = 45 cm) was larger than the dbh of hemlocks used for feeding only (x̄ = 33 cm) and was also larger than a random sample (x̄ = 35 cm). Porcupine selection of tree species varied between study areas according to availability of tree species. Their selection of den types also varied between areas according to availability. This study shows that porcupines are adaptable during the summer but are more restricted in their habitat use during the winter. The loss of hemlock may increase mortality in this population due to winter stress and predation.