The main aim of this field study was to compare adult male and female grey mouse lemurs (Microcebus murinus) with respect to body size, seasonal changes in body mass, onset of reproductive activity and sex ratio of captured animals. M. murinus was studied in the dry deciduous forest of western Madagascar. The lemurs were trapped in capture/recapture sessions during the dry season of 1993 (August, September, October). In order to verify results of studies in captivity concerning body mass and rank order among males, distribution patterns and body mass of males and females were examined. The results of this study indicate that rank in males is related to body mass and testes size, rather than determined by body size. Heavier males occurred closer to females, possibly facilitating access to breeding opportunities. There was no sexual dimorphism in body size. Differences between the sexes in seasonal changes in body mass, and in numbers of each sex caught, indicate that males and females have different activity patterns during the dry period, when food availability is restricted.