Catocala fraxini is a moth of the family Erebidae. It is commonly known as the blue underwing. It was initially described by Carl Linnaeus in the 10th edition of Systema Naturae, published in 1758. The species is endemic to most of central Europe, northern Europe, and parts of southern Europe. It is found in much of the Palearctic, Turkey, Siberia, the Russian Far East, Korea, and Japan. It is absent from Portugal, the Mediterranean islands, Greece, northern Scotland, northern Scandinavia and southern Russia.

The forewing is white and yellow. The pattern is irrorated with pale or dark grey colouration, which sometimes features a yellow accent. The inner and outer lines are dark. These are serrated and bilateral. The median and subterminal lines are dark and serrated. The reniform stigma features a black centre and outline. There is a pale diamond-shaped yellow pattern outlined with black beneath this. The hindwing is dark, with a broad blue postmedian band.

The larvae are brownish grey and black accented. These have pointed sections on the 9th and 12th segments.

The caterpillars feed on various species of poplar, or other species within the Populus genus.

Two subspecies of this taxon exist. The Adscita statices statices is found in Spain, in most of central and eastern Europe, in Siberia, Turkey, and China. Adscita statices drenowskii is found in the Balkans and Turkey.

The topographical distribution of each subspecies ranges from dry, sandy, and calcareous habitats to montane locations, and wet meadows and marshes.

Both subspecies are visually similar to the Scarce Forester (Adscita globulariae) and Cistus Forester (Adscita geryon). Adscita statices are often found feeding at flower species, including those within the taxon Rumex. Devil's-bit scabious, field scabious and marsh thistle are further frequent.

The wingspan of the species is 25 to 28 millimetres in length. The antennae, head, thorax, legs and abdomen are vibrant green, although the thorax can be occasionally blue toned and iridescent. The wings may appear rusty-red in the early morning and evening due to its coloured scales, which feature pores that can fill with water and visually change in hue. The wider forewings are less glossy than the body. These are mostly blue green, but can also be green to yellow green. The edges of the wings are black-grey. The hindwings are moderately wide. These feature a slightly truncated rim and are slightly transparent, with a blackish grey hue that is lightened basally. Both forewings and hindwings have a grey underside.

The caterpillars are pale yellow-green. There is a central dark stripe vertically across the larvae, and white hairs on the dorsal sections.