Lepilemur edwardsi and Avahi occidentalis are both small, nocturnal folivores, but live sympatrically over a large part of their range, suggesting that niche differentiation has occurred. In order to establish whether ranging behaviour and activity patterns have contributed to niche differentiation, a field-study was carried out at Ampijoroa, Madagascar, within the area of range overlap. Four individuals of each species were fitted with radiocollars and tracked, dusk to dawn, over 18 months. Data were collected on home-range size, travel distances, activity and social behaviour. Neither showed the markedly biphasic activity patterns which have often been described for nocturnal prosimians; this might be explained by the combination of folivory and small body size. L. edwardsi spent long periods inactive, but engaged in considerable social interaction during the night, including extensive long-range calling. The mean home range was 1.09 ha, and ranges were rather evenly used. Home ranges of females overlapped extensively, and the range of the single adult male which could be caught overlapped with the ranges of several females, as is common in nocturnal prosimians. The mean nightly travel distance was very low, only 343 metres. On the other hand, social interactions in A. occidentalis were primarily within a stable monogamous family unit, and long-range calling was very limited. Home ranges were rather larger than in L. edwardsi, but less evenly used, and mean nightly travel distances were much greater, 1175 m. Differences in range-use patterns may be explained by the contrast between Avahi's, selective feeding on relatively high-quality young leaves and flowers, versus Lepilemur edwardsi's, unselective feeding on relatively low-quality, but ubiquitous and evenly-distributed, mature and old leaves. Similarly, while overlapping female ranges may be adequate when food resources are evenly distributed, monogamy in Avahi may be a response to selective feeding on a patchy food resource.