This strikingly coloured species reaches a length of 10-15 cm or more but is only 1-4 mm wide. The bluntly rounded head, which tapers anteriorly, bears distinct cephalic slits and 3-12 small reddish-brown to blackish eyes arranged in a row near each cephalic margin. Eye number varies with size and age; in juvenile worms with a length of about 1-2 mm there is usually only a single pair of distinct eyespots situated near the tip of the head. The body is somewhat flattened ventrally and gently tapers posteriorly to end in a pale caudal cirrus, which may be quite long.
The colour pattern is characteristic for the species. Dorsally, Micrura fasciolata is usually a rich reddish-brown hue marked with regularly or irregularly distributed white transverse bars, but the background colour may be brown, yellowish-brown, greenish-brown or red-violet. Sometimes the white bars are indistinct or lozenge-shaped. The tip of the snout and the cephalic slits are white. A dorsal but very variable patch of pigmentation is present on the head, which may be entirely coloured (apart from the white tip) or be predominantly white. The cephalic pigment may also be slightly darker than that of the remaining dorsal surface. Ventrally, the species is a much paler colour, although still tending to be reddish-brown and marked with faint lines or furrows which are continuous with the white dorsal bands (M. fasciolata). Younger individuals generally possess a paler colouration, and often the posterior portion of the body is uniformly tinted without transverse markings. Dermis with a distinct connective tissue layer. Proboscis with two muscle layers and two muscle crosses. Cephalic glands are well developed.
Often locally abundant in sublittoral regions, dredged at depths to 80 m or more, but may occasionally be found intertidally near the low water level. The species has been found in a wide range of sediments, including sand, gravel, shell fragments and mud but is also commonly associated with tubicolous polychaete tubes, especially Pomatoceros , laminarian holdfasts and Zostera beds or may be found in rocky fissures or beneath boulders and stones. Sexually mature individuals are generally obtained during the September to December period, rather later in more northerly latitudes.
The geographic range of Micrura fasciolata extends from Scandinavia to the British Isles and the Mediterranean.