The daily onset of activity in fireflies and glowworms is a complicated process involving both a circadian rhythm and a triggering mechanism controlled by ambient illumination. Onset of the luminescent activity has been investigated in the field and under semi-natural and experimental conditions. 1. the onset of flashing or glowing occurs at a critical low illumination during or just after the twilight period. This was determined in the adult glowworm Lampyris noctiluca in Denmark and in the fireflies Photuris congener and Photinus umbratus in Florida. 2. The dispersion of activity onsets differs in the three species which is partly due to differences in duration of the decrease of illumination (duration of twilight). At the long duration of twilight in Denmark, L. noctiluca has a greater dispersion of onsets than has P. congener at the short twilight in Florida, although the period of activity onsets occurs during about the same range of illuminations. 3. The onset of activity occurs at a lower illumination at low than at high temperatures. 4. At a reduction in illumination to different low values an increasing number of animals does not commence activity above about 1 lux. There is no activity if the illumination remains above 10 lux. 5. A period of latency appears in the onset of luminescent activity by a sudden change in illumination from light to dark (dim light). The duration is about 11 min in L. noctiluca, 5 min in three species of Photuris and 3 1/2 min in P. umbratus. The latency is constant at different illuminations below the threshold for overt activity. The process underlying the latency also occurs at illuminations above the threshold, but it is slowed down the higher the illumination. Further, the latency is shorter at a fast decrease of illumination than at a slow. 6. It is suggested that onset of activity in nocturnal insects involves three steps. A circadian rhythm of sensitization brings the animal into a specific state of receptivity. This happens close to the normal time of activity onset. Further, a latency process controlled by illumination occurs before overt activity can take place. This is then released below a certain threshold level of illumination.