The sexes appear similar when at rest with the wings closed, and also in flight when they appear as pale almost silvery blue, but this is a little deceptive and male and female are different on the upper side. The upper side markings of the male are a deeper blue, with a fine black border to fore wings and a white edging fringe, through which the black veins pass. The hindwings have only the merest black line edged with the white fringe. The female is more of a true blue on the upper sides with a heavy black band across the forewing tips and borders and the white fringe and six small black dots on the hindwing border. The black shading is wider, heavier and more intense in the second summer brood. The undersides of both sexes are a paler shade of blue with very small black dots, quite different from any of the other British blues. The female has 11 dots on the underside of the hindwing in the spring brood and 15 in the summer brood.
The spring generation emerges from March and can be seen flying until mid-June. The second generation emerges in late July and August.
The Holly Blue does not visit flowers as much as other blues, but will come down into sunny meadows, especially the second brood, onto Marjoram. It will settle on damp ground to sip water in dry weather, and takes trace elements and vitamins from horse droppings, and also bird droppings on the leaves of trees. In good locations, it is quite common to see dozens of Holly Blue together on the ground at a favourite puddle or pile of manure.