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This is a very scarce butterfly, restricted to marshy meadows where the foodplant, greater burnet, grows in abundance. Where it flies, it is often very numerous. The problem is, its favoured habitats are themselves threatened and fragmented, especially in the western parts of its range. In Switzerland, I know several places where in late June and early July almost every head of burnet seems to sport either this species or its relative, the dusky large blue, another butterfly associated with this plant. Equally, I know whole cantons where it is impossible to come across either species at all.

In appearance, the scarce large blue male is similar to lowland large blues, except that the dark forewing upperside spots are smaller - often vestigial or absent. The female is more like the upland forms of large blue, being heavily suffused with dark, though also with small or vestigial post-discal markings. On the underside, both sexes are grey brown with dark spots circled in white. Where large blues have two rather clear rows of dark submarginal spots, its scarcer relative has one, the inner one, with just the faintest shadow of spots outside this. The general appearance of the underside is altogether more subdued. The presence of greater burnet is a necessary condition of finding a population of scarce large blues, and as already noted, females in particular are almost always to be found on or near this plant. However, large blues often share the peripheries of the same habitat and the two species may fly together. Since alpine large blues may be heavily suffused with dark scales, older specimens should be observed closely to confirm the presence of scarce large blue. A similar, dark form of large blue flies in the Pyrenees and I suspect this is responsible for some of the alleged sightings of scarce large blue there. It is less likely, but not impossible, that you will find alcon blue in the same habitat. I have never yet come across a site where this happens.

All Phengaris species have the same intimate relationship with ants. The third instar caterpillar must be collected by ants and taken down into the nest, where it becomes carnivorous, feeding on the ant larvae. It pupates in the nest and spends the winter there, hatching and clambering to the surface to fly the following summer. The presence of the correct species of ant is another limiting factor governing the distribution of this butterfly.