Activity of fast growing broiler chickens is known to decrease at two to three weeks of age, whereas slower growing hybrids are not only more active, but remain so throughout the growth period (Reiter and Bessei, 1998). The present experiment aimed to monitor diurnal patterns and time courses of activity in groups of commercial broilers fed one of two feed types differing in energy content. Activity was assessed using passive infrared detectors (PIDs), which sense movement of an object with a temperature different from that of the background.