The fan activity rhythm of Sabella spallanzanii (Gmelin, 1791) and its entrainment capability to light were studied. Animals were tested under constant darkness (DD) followed by two consecutive 24 h light-darkness regimes: a first 11 h light period (LD) and a second 9 h light period, with its phase inverted (DL). An infrared analogical video-camera took shots each 30 s. A number of pictures with open fan were counted every 15 min. In DD a weak free-running periodicity in the circadian range was found, thus reinforcing the matching of the 24 h period under study in both photoperiod regimes. A nocturnal activity was characterised with a consistent anticipation to lightOFF (i.e. entrainment). Moreover, this phase of entrainment differed between DL and LD. The presence of endogenous activity rhythm with a variable phase angle of entrainment is a distinctive feature of circadian pacemakers.