Mesopelagic crustaceans occupy a dim–light environment that is similar to that of nocturnal insects. In a light–limited environment, the requirement for greater sensitivity may result in slower photoreceptor transduction and increased summation time. This should be reflected by a lower temporal resolution, as indicated by a lower critical flicker fusion frequency (CFF). Therefore, one would predict that the CFFs of mesopelagic organisms would be relatively low compared with those of their shallow–water relatives, just as nocturnal insects tend to have lower CFFs than diurnal insects. Using an electrophysiological apparatus that was adapted for shipboard use, the dark–adapted CFFs of a variety of species of mesopelagic crustaceans were determined using the electroretinogram. The parameter examined was the maximum CFF—the point at which further increases in irradiance no longer result in a faster flicker fusion frequency. The results summarized here indicate that there is a trend towards lower CFFs with increasing habitat depth, with some interesting exceptions.