The ecology and status of the declining butterfly, L. bellargus, were studied by recording the behavior of all stages of the life cycle; by monitoring adult population changes on 7 sites; and from surveys and habitat analyses of former and existing colonies. L. bellargus forms local closed populations that fly in small discrete areas. It reaches its northern limit in Britain, and breeds mainly on south-facing slopes in the extreme south, although its foodplant, Hippocrepis comosa, is common in most calcareous grassland. The larvae are tended incessantly by ants after their 1st instar. Ants also tend pupae, which occur inside their nests or are buried in earth cells. Egg-laying is virtually restricted to H. comosa growing in a 1-4 cm tall sward, perhaps because these situations are warm and support many ants. Monitored populations were larger when their sites were closely-cropped, although very heavy grazing was harmful. No colony was found on a former site whose sward had grown above a mean height of 5 cm. The number of L. bellargus colonies in Britain has halved every 12 yr since the early 1950s. Perhaps 70-80 populations survive, of which most are small. Most (perhaps all) extinctions were due to habitat change. About 1/3 resulted from the loss of H. comosa, mainly through ploughing or agricultural improvement. All other former sites support an abundance of foodplant, but few are sufficiently well grazed nowadays to support L. bellargus. Butterfly collectors, colony isolation and bad weather are unlikely to have been important factors in the decline. It is often uneconomic to graze unimproved grassland adequately for L. bellargus; nor, since myxomatosis, have unstocked sites been maintained by rabbits. Most remaining colonies will become extinct unless conservation bodies intervene. Past measures have proved inadequate, and new advice is given.