The diel activity patterns of Helicopsyche murrumba (Trichoptera) were observed over two 24 h periods in the laboratory with either normal or reversed lighting regimes. During normal conditions, activity reached a peak 3 h after sunrise (S.R. 0600 h) and remained active until 2 h before sunset (S.S. 1800 h) With stepwise reduction of illumination from > 250 lux to < 0.2 lux over an artificial 2 h “sunset” period the larvae moved back onto rocks and the level of activity gradually declined. No apparent stimulus to move under rocks was observed. Under reversed lighting conditions the periods of high and low activity were also reversed. It is suggested that an endogenous rhythm of activity is absent in H. murrumba and that diel activity is controlled by a direct response to light intensity, an exogenous component.