AbstractEvidence of a circadiam clock mechanism was found in the cave crayfish, Orconectes pellucidus, even though it is blind and has been isolated from daily cycles for at least 25,000 generations. Five of seven crayfish tested under cave conditions of continuous darkness and 13°C showed apparent rhythms of activity and/or oxygen consumption. These rhythms were statistically significant, as judged by autocorrelation, for three individuals with average periods of 26, 27, and 34 hours. All three crayfish exposed to light:dark cycles with a 24 hour period showed more regular rhythmicity than in continuous darkness but only one showed clear evidence of entrainment, i.e., synchrony of activity or oxygen consumption with the light: dark cycle. Another crayfish tested apparently conformed to Aschoff's circadian rule.