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.