- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Hairstreaks – Theclinae
- Wing span: Very small, 20–26 mm (0.78–1.01 in.). Females slightly larger than males.
- Wing upper side: Brown.
- Wing underside: Green. Hind wing with white row of blotches.
- Habitat: Spruce and mixed forests, bogs, slopes.
- Flying time: Early May–Mid-June. Slightly later in the north, until early July..
- Overwintering form: Cocoon.
- Larval foodplant: Different kinds of Heather plants (Ericaceae) such as bilberry (Vaccinium myrtillus), cowberry (V. vitis-idaea), bearberry (V. uva-ursi) and bog bilberry (V. uliginosum). Also tufted vetch (Vicia cracca) and zigzag clover (Trifolium medium).
The green hairstreak is one of the most common butterflies in Finland and is the only unspurred hairstreak. It is the first of the butterflies that overwinters as a cocoon to emerge in spring (those that overwinter as adults are earlier). Like other hairstreaks the green hairstreak rests with its wings pressed together, which means that the upper side can only really be studied when the butterfly is in flight. It is quite easy to recognize, however, because it is the only butterfly in Finland with green wing undersides.
Males wait for females in their territories in forest margins and clearings. The females lay their eggs individually in the host plant’s flower buds.