The present paper describes the population parameters and natural history of Heliconius hermathena hermathena (Hewitson, ), a sand forest specialist nymphalid butterfly. Population biology was described based on a 14-month mark-recapture program in a site of open forest in Pará state, northern Brazil. The population was constant through the year, with no marked peaks of abundance for both sexes, with females always less abundant. The range of the population size was 70–150 individuals (with a maximum near 200 individuals). Sex ratio was male biased, with males dominating most of the time. Average residence time was of 35 days for males and 31 days for females, with a maximum of 139 days recorded for males and 129 days for females, with both sexes presenting survival curves approaching the type II survival curve. Males presented wing sizes greater than females in all months. Adults were observed visiting five species of flowers as nectar and pollen sources and establishing communal nocturnal roosting aggregations on small shrubs. Considering the fragility of the Amazonian white sand forests, understanding the population patterns of H. h. hermathena can help future conservation planning for these potentially threatened habitats.