Lemur fulvus mayottensis was observed on the island of Mayotte from January through May 1975 (wet season), and from July through August 1977 (dry season). Study groups were scan-sampled for activity at 5-min intervals, and daily activity patterns were derived from these figures. The daytime activity:rest ratio declined from the wet season (0.52) to the dry (0.40), although a general pattern of early morning and late afternoon activity, with a substantial period of midday rest, was maintained. Such a decline also has been noted for L. f. rufus in Madagascar; in both cases it coincided with an increase in the proportion of leaves in the diet. During the dry season, nocturnal as well as diurnal observations were made; these revealed that at least as much activity occurred at night as during daylight. No appreciable difference existed between night and day in either the type or the intensity of activity, or in the food sources exploited (indeed, the two major feeding bouts began, respectively, before dawn and sunset, and continued after each). Such 24-h activity has not been reported in wild-living primates, and its possible significance is considered.