The sexes have different upper surfaces to the wings but fairly similar underwings. The male has a pale powder blue upper wing surface which may vary from a very pale silvery blue through to a greenish tinted blue according to locality, although it always carries the outer wing margin of a blackish band with six or seven partly submerged black dots to each wing, completed by a narrow white outer margin. The female is dark brownish-black or soot colour, and her rows of black dots have very slightly orange-tinted surrounds, followed by a dingy white outer border. The under sides of both sexes are respectively greyish and in the female pale brownish. Both have 10 black, white-circled dots on the forewings, and 12 on the hind wings, then a border of black dots, faintly orange-circled in the female.
Males emerge first, usually in mid-July, and if the weather is suitable with opportunities for regular nectaring, he will live for seven weeks. Quite often though, August is the only month one can be sure of seeing the Chalkhill Blue.
A colony will often centre around a favourite stand of Marjoram, to which they will all go at regular (hourly) intervals for refreshment. Otherwise, mainly low growing herbs are used, especially Vetches, Thistles, and Hawkbits.