Dainton (1954) showed that activity was stimulated by falling temperature below 22 °C in the slug Deroceras reticulatum (Agriolimax reticulatus) and that this response could account for the timing of the activity observed in the field. Runham & Hunter (1970) state that Lewis (1967) was unable to repeat this result on Arion ater. Experiments described here show that this slug does respond to falling temperatures below 22 °C by becoming active and that the time between the onset of the stimulus and the appearance of the activity is related to the rate of fall. The apparatus consisted of a stirred water bath of uniform temperature which could be lowered at controlled rates by regulating the influx of iced water. Immersed in the bath were six air-filled glass chambers, the floor of each of which was lined with wet filter paper which served the dual purpose of saturating the air with water vapour and providing the slug in the dish with a wet surface from which it could absorb water and so replace that lost in mucus production during locomotion (Dainton, 1954). Each chamber contained one slug and even after 5 days (more than twice the duration of any experiment) all slugs were in excellent condition. Before the experiments slugs were kept in glass dishes, fed on vegetables and subjected to the diurnal changes of temperature (16—22 °C) and light intensity of the laboratory, when feeding and locomotion occurred only at night. Introduction of the slugs into the apparatus during the day stimulated a short period of activity never exceeding 1 h, and no experiment was begun until activity had ceased. About half the experiments were performed on slugs which had been in the apparatus overnight. There was no difference in the results obtained on the first or second day in the apparatus. A thermistor inserted through a glass chimney into the experimental chamber registered the same temperature as that of the water bath, and its timedependence was also the same except at the higher rates of fall. These calibration data were used to give the corrected temperature at these higher rates.