Abstract Electroretinograms (e.r.g.s) of compound eyes and dorsal ocelli in three diurnal insects— Apis mellifera, Pachydiplax longipennis, and Phormia regina—have high flicker fusion frequencies in the range 190/sec to 350/sec. The e.r.g.s of compound eye and ocellus in one nocturnal insect, Periplaneta americana, have flicker fusion frequencies in the range 45/sec to 60/sec. Flicker data for the compound eyes confirm to the earlier observations of Autrum (1950). Autrum's suggestion that fast eyes (those with high f.f.f.) may be further characterized by a typical e.r.g. wave form, low photic sensitivity and extremely rapid rate of dark adaptation lacks generality. The honeybee compound eye (f.f.f.=265/sec) has an e.r.g. quite similar in wave form to that of the cockroach compound eye (f.f.f.=60/sec). None of the eyes studied has higher sensitivity than the dragonfly ocellus, an eye which has a flicker fusion frequency of about 200/sec. This same ocellus dark-adapts no more rapidly than the cockroach compound eye. Wave form of the e.r.g., photic sensitivity, and rate of dark adaptation are independent visual functions, not necessarily related to each other or to flicker fusion frequency. The data suggest that the ability to resolve high rates of flicker depends on properties of the retinula cells themselves and not on properties of postsynaptic electrical events as suggested by Autrum (1952).