The larval luminescence in L. noctiluca consists in a glow lasting for several seconds (average 7.3 sec.) emitted at irregular intervals while the animal is crawling about during the active period at night. In the adult female the glow is continuous often for several hours. Also some larvae in the last instar were able to glow continuously for hours. Onset of activity in the larvae occurred at a lower critical ambient illumination (6.85 log lux +10) than in adult females (10.10 log lux+10). The dispersion of onsets was greater in larvae than in females. The duration of luminescent activity in the field was ca. 5 hours and at experimental conditions ca. 8 hours. In thc field larvae were mostly seen during late summer, probably because of the low critical illumination, which was also the reason why activity was suspended on moonlit nights. The intensity of moonlight around full moon was ca. 8.00—9.00 log lux+10. In the discussion the following subjects are treated: The function of larval luminescence; the reaction of larvae to ambient illumination; and the difference between glowing and flashing.