1. Herring (Clupea harengus) require light for feeding and avoiding nets. The threshold light intensity for these behaviour patterns and also for phototaxis, using different colours of light, was measured and used to obtain spectral sensitivity curves.
2. The maximum sensitivity of dark-adapted herring lay between 510 and 520 mµ. The threshold light intensity, using white light, varied from 3.6 x 10-2 to 2.9 x 10-4 photopic lux, depending on the behaviour being studied.
3. A parallel experiment on a dark-adapted human subject gave a maximum sensitivity at 500 mµ and a threshold light intensity, using white light, of 3.4 x 10-7 photopic lux.
4. Pigment extracted from dark-adapted eyes of herring had a maximum absorption at 500 mµ.
5. Suggestions are made for measurement of underwater light intensity in studies on fish behaviour.