Castalius rosimon, the common Pierrot,[1][2][3][4] is a small butterfly found in India that belongs to the lycaenids, or blues family.

The species is found in Sri Lanka, Bangladesh, India,[1] Myanmar; Tenasserim, extending into the Malayan subregion.

In the Indonesian archipelago the butterfly occurs in north-eastern Sumatra, Kalimantan, eastern Java, Bali, Bangka, Timor, Wetar, Kissar, Sumbawa and Sulawesi.

[5] Individuals of this species have been reported to exhibit seasonal colour variations and melanic aberrations that shows a similar pigmentation pattern.

Forewing has a long oblique black band from base outwards to the costa; below it and obliquely placed an irregular black somewhat conical mark; following these are two outwardly oblique, medially interrupted, black macular bands; the inner of the two extended from costa along the discocellulars, is then widely interrupted below its posterior portion that is formed of two elongate coalescent spots and touches the inner subterminal transverse line of elongate spots just above the tornus; the outer, obliquely placed line is subapical and medially broken, the middle portion consisting of a quadrate spot is shifted outwards; finally, two parallel subterminal transverse series of black elongate spots, the inner series of broad, more or less rectangular spots, the outer series of more linear spots, the latter coalescent anteriorly with a slender anteciliary black line.