A capture-recapture study was carried out on an obviously isolated population of Heodes virgaureae in southern Sweden during 37 days (=male flying season) in 1964. The two sexes were poorly synchronized in time; the first male was seen 8 days before the first female and the latter reached their maximum numbers when the male numbers were already declining. Gain to and loss from the male population was estimated and it could be demonstrated that most of the gain and loss occurred during periods of warm and sunny weather, i.e. during periods of high activity in the butterflies. Moreover it was shown that the loss rate was higher between morning and afternoon than during the rest of the 24 h period. The mean daily survival rate was estimated at 0.89 in the males. By moving along a route that covered the entire area and counting all observations of the butterfly, area censuses were carried out which were checked against the capture-recapture estimates. The censuses made in the morning during favourable weather conditions, i.e. more or less sunny and not too hot or too cold, yielded good estimates of the population.