The otter (Lutra lutra) population in the De Onlanden Nature Reserve (Province of Groningen, the Netherlands) has been monitored for four years using seven cameras placed at locations where otters regularly passed. Analyses of the recordings of these cameras showed that otters were present and active throughout the year and were mainly active in the dark period of the day. Males were found to be most active in the late summer months, while family groups were seen mostly in the winter months and were absent in the summer period. During the long winter nights, activity was spread out over the whole night and family groups and ‘other’ otters continued to be active at a low level in the daytime hours. Males were not active in daylight. Both males and family groups seemed to have short breaks during their night time activity. In the short summer nights the otters started being active a little (1-2 hours) before sunset and were most active at the start of the dark period. Activities diminished gradually during the night, continuing until approximately one hour after sunrise. Almost no activity was recorded during the long daytime period in summer. Comparison of locations with and without human disturbance (i.e. traffic) showed that in the absence of human disturbance the otters (especially families) were significantly more active during the daytime period.