The sleep-waking cycles of the three-toed sloth, Bradypus tridactylus, were studied by direct observation and polygraphic recording and the results were correlated. Three states of waking behavior were recognized, "awake-exploring", "awake-alert" and "awake-fixating". EEG recordings were classified as two waking states., A1 which was seen during both "awake-exploring" and "awake-alert" behavior and A2 which had some of the characteristics of drowsy behavior of other mammals and often accompanied the "awake-fixating" behavior. EEG patterns of sleep were divided into five types: two in light sleep, two in deep sleep and one in paradoxical sleep. Animals spent about 30% of the time in various degrees of waking behavior and about 70% asleep. Waking EEGs of various types were seen about 34% of the time, about 10% of that during behavioral sleep. EEG patterns of light sleep constituted about 56% of the records, about 6% of that during some form of waking behavior. EEG patterns of deep sleep constituted about 10% of the records; all of this came during behavioral sleep. The total duration of sleep and of paradoxical sleep did not depart materially from the expected pattern based on phylogenetic position. The pattern of sleep is polycyclic but in the laboratory situation animals were sleeping most soundly between 6 a.m. and noon.