In some natural populations of Cyclops bicuspidatus thomasi and Canthocamptus staphylinodes the ontogeny of each species includes an encysted resting stage within a definite instar. The factors inducing encystment are obscure. The presence or absence of the resting stage in a population can not be correlated with lake typology or with any of the obvious environmental physicochemical conditions. Excystment can be induced by placing cysts in oxygenated water, yet natural excystment does not necessarily coincide with lake overturn periods. Excystment is inhibited temporarily by an anaerobic medium, but the cope pods emerge eventual1y in such an environment. Both species survive long periods in anoxic sediments, and are believed to be capable of anaerobic respiration. There is limited evidence that Cyclops is resistant to respiratory poisons during part of the resting stage, and may retain the resistance for at least two weeks after excysting, if kept within the poison solution. Newly excysted individuals in an aerobic medium become increasingly susceptible to respiratory inhibitors as time passes even though in the same instar. Active Canthocamptus individuals from profundal sediments are paralyzed by cyanide, but survive 3 or 4 days in a solution which quickly kills planktonic copepods of other species. Physiological and ontogenetic features of copepod encystment suggest that it may be defined as diapause, a word which has been applied in most cases to insect phenomena.