Large populations, seemingly not at risk of extinction, can decline rapidly due to alteration of habitat. This appears to be the case of the butterfly Chazara briseis, which is declining in all of Central and Eastern Europe, even from apparently large areas of its steppe grassland habitats. We combined mark–recapture, allozyme electrophoresis and adult behaviour observation to study the last remaining metapopulation of this once-widespread butterfly in the Czech Republic. The total population estimate was 1300 males and 1050 females in 10 colonies within a 70 km2 landscape. Adults were long-lived, and inseminated females required several weeks before they started ovipositing. Models using realistic lengths of the preoviposition period estimated that due to background mortality, only 25–55% of the female census population lived long enough to contribute to the next generation. This demographic load was unlikely to be balanced by an increased fecundity. Allozyme electrophoresis of 22 loci revealed much higher allelic variation than in most other studies of butterflies living in small populations (mean heterozygosity: 20.7%). If expressed as per individual colony, the genetic variation did not correlate with population density, survival or longevity. This was probably due to frequent movements among colonies; during 8 weeks of adult flight, 5.1% of recaptured males and 3.6% of recaptured females moved between colonies. The high preoviposition mortality indicates that populations of this species must contain more individuals compared with populations not suffering this additional demographic load. The high allelic diversity of each single colony suggests that the population as a whole has not undergone genetic bottlenecks, but now may be facing risks of inbreeding depression due to allele frequency shifts and the possible increase of weakly deleterious alleles. In the past, high effective population sizes were maintained by frequent dispersal in dense networks of steppic grasslands. Generous habitat restoration is necessary to safeguard populations of this specialized, yet formerly common species.