The activity of radiotagged adult crested porcupines Hystrix cristata L., 1758, was studied in a rural hilly area of Central Italy, from July 1990 to June 1991. Three males and one female were monitored every 15 minutes for 18–24 hours/individual/week, and their activity patterns were correlated to ambient temperature, precipitation and lunar phases. Three individuals, out of the four monitored, showed a fall of activity in the beginning of the cold season, but generally porcupines appeared adapted to a wide spectrum of temperatures. The mean duration of nocturnal and crepuscular activity was 9 h 22'±42', showing little variation throughout the year. In the cold months, i.e. when nights are relatively longer, porcupines usually left their burrows after sunset and came back some hours before sunrise. In the warm season they left their burrows at sunset, or shortly before, and returned at sunrise, or shortly after. Overall diurnal activity was scarce (2 h 11' ± 1 h 30'), but in spring it increased to over 40% of the diel activity (3 h 22' ± 3 h). Moonlight avoidance was slight. Precipitation did not seem to influence the activity of porcupines.