In I968 and I969 noctuids were caught by means of a light trap at Bunkeflo, south of Malmo (prov. Scania). Hourly catches were separated. Sex differences in the diel distribution of catch were found. In summer, the males showed their highest activity around midnight. The females then showed a bimodal catch curve with the highest catch before, and a second weaker peak after midnight. In summer, light was found to be the factor governing the nocturnal distribution of catch, in autumn temperature. In I970 observations and experiments on captured moths were performed. Start of flight in the twilight period was found to be induced by the decreasing light. The moths started at higher light values if the temperature during this period was high. In actograph experiments it was found that moonlight has a depressing effect on the activity of noctuids.