In the past decade there has been an increasing awareness that spontaneous activity in hibernating mammals is not a mere accidental imperfection in the mechanism of hibernation but, on the contrary, a regular and important feature of it. Several recent investigations (e.g. Kristofferson & Soivio, 1967; Pohl, 1967) have dealt with activity of mammals hibernating under laboratory conditions. Comparatively little is known of activity patterns during the hibernation season in natural circumstances. In the framework of studies on the ecological factors involved in activity during hibernation field investigations were made in several winter quarters of bats. In these investigations, a complete account of which is to be published elsewhere, two methods have been used to measure rates of activity in the populations. One is the constant interval search with calculation of the relative number of bats which have moved from their locations each interval (Daan, in press). The other method involves photographing active bats as they fly through a passage in the hibernaculum. A photocamera was used in preference to any other form of recording in order to be able to distinguish between species and even between individuals. The equipment developed will be described in the present paper and illustrated with a few preliminary results.