During a study of a small, relatively isolated population of Glaucopsyche (Maculinea) nausithous (Bergsträsser, 1779), nearly every new adult entering the population was individually marked in summer 1990. Marked individuals were regularly recaptured. The population assessment was nearly complete, as shown by a comparison with estimates based on the Jolly-Seber-model. The average adult residence time was estimated at 2.3 days. Individual observations were made of the diurnal distribution of the following activities: resting, flight, nectaring, oviposition, copulation, and [other] movements. Our results show sex and age specific differences. On the day of marking, which was normally the day of eclosion, all activities were comparatively low. Older individuals of both sexes were generally active for the same amount of time per day, but differed with regard to activity profiles. Males undertook longer flights to search for females within the habitat, while the females flew more often but over much shorter distances, resulting in a shorter total flight time. Instead, females stayed on the flower buds of Sanguisorba officinalis the only larval host plant and main adult nectar source for much longer and laid eggs. Females copulate directly after hatching and then immediately start to oviposit.