Geitoneura klugii klugii (Guerin-Meneville) and G. acantha ocrea (Guest) coexist over much of south- eastern Australia. Their adult phenology, sex ratios, population sizes, longevity and habitat distribution were compared at several sites near Melbourne, in 1985-86. Both species are univoltine and protandrous. G. klugii appears slightly earlier in the season than G. acantha. Both sexes of G. klugii remain active throughout their flight period, whereas female G. acantha apparently undergo a short inactive phase after mating. Sex ratios appear male-biased (total captures, G. klugii 85.4% male; G. acantha 65.2% male), although this became less pronounced as the season progressed. Regular samples, and mark- release-recapture studies, showed a clear transition from fresh to worn individuals through the season, although females remained fresh until later than males. There is some suggestion that male G. klugii 'hill-top' towards the end of their flight season. A range of ecological strategies apparently occurs in coexisting species of satyrines in south-eastern Australia. The biology of Geitoneura is compared with that of Heteronympha merope (F.), and with satyrines elsewhere in the world.