We collected data during a 10-month study carried out on the mongoose lemur, Eulemur mongoz, at Anjamena in northwestern Madagascar, which provide baseline information on seasonal variation in the ecology, home range use and some aspects of the behavior of two neighboring groups. We monitored group size of nine groups in the study area and assessed them for seasonal variation. We present additional information collected during short-term surveys in other areas before and during the study for comparison. The study groups were small family units, and changes in group size were limited to births and emigrations of sexually mature progeny. In spite of clear seasonal changes in climate and vegetation, there is no variation in grouping patterns, so it is not possible to correlate variation in group size with seasonal variation of ecological variables. Comparison with ecological data from other field studies on lemurids reveals differences in food resource distribution in western forests versus other types of Malagasy forest. This distribution of food resources may predict home range size in mixed frugivorous–folivorous lemurs. Small home ranges, mainly in the West, could be correlated with a uniform distribution of food resources. Finally, we suggest that the dry season in the West may not present frugivorous–folivorous lemurs with major problems in finding an adequate food supply. This is supported by the lack of seasonal differences in ranging behavior of mongoose lemurs.