Observations were made on the diet, activity patterns, social organization, ranging and vocalizations of Avahi laniger laniger, a nocturnal prosimian, in rain forest in eastern Madagascar. A male woolly lemur was radio-collared for a period of one month and most data were collected from follows of this individual. During this time, all observed feeding was on leaves and only leaves were found in faecal samples. Resting and grooming occupied almost 65% of each night and this inactivity is probably related to the low quality diet. Avahi were most commonly seen in pairs; the radio-collared male almost always slept with a single female and he spent 40% of the night with her. An infant was born to the study pair in August. Range size was 1·4 ha and this appeared to be used exclusively by the male and his female. Avahi seems to be a very unusual primate; neither folivory nor monogamy is common among the nocturnal prosimians.