To examine the way in which light intensity and spectrum affects the swimming behaviour and activity of the pelagic euphausiid Meganyctiphanes norvegica. Our initial objective was to develop a method with this animal where clear behavioural responses could be related to various stimuli. By tethering the animals it was hoped that it would be possible to look at the responses of Meganyctiphanes norvegica to subtle changes in light intensity, of the range they might be expected to experience in their natural habitat. Concomitant with the main objective, animals were sampled over 24 hours to look for the presence of clock proteins and examine the movements of visual pigments. To relate any pigment migration to changes in light intensity that the animals might have experienced in situ, animals were also exposed to known quanta of light and then fixed.