The culpeo fox is the largest native canid in Patagonia. The majority of past studies on culpeo foxes have focused on trophic ecology, population dynamics, natural history, competitive interactions, management, and habitat use. Little work has been done on determining activity patterns of the fox and thus, conclusions regarding culpeo fox activity patterns have been based on limited data. The objective of our study was to determine culpeo fox activity patterns in a protected area (Lanín National Park, Argentina) using “camera-traps” to test the hypothesis that the culpeo fox is still nocturnal in non-hunting areas. Data were collected from October 2008 through May 2009 at 29 infrared triggered camera stations. We obtained 1,261 culpeo fox photos of which 234 were used for analyses. Diet was studied from 34 culpeo fox scats. Culpeo foxes were most active during nighttime (>70% of the records) confirming the general patterns obtained in other studies. Additionally, examination of culpeo fox scats revealed that they mostly preyed on nocturnal small mammals. The present study offers evidence against the widespread assumption that nocturnal activity of this species is a behavioral response to human harassment, as it was conducted in a protected area where culpeo foxes are not hunted. Instead, their nocturnal behavior may be related to prey activity patterns.