The size of the slug varies from 10 to 15 cm. Maturity is reached at about 2.5 cm. The color of the black slug is generally deep black, but the colouration is very variable and this slug can even be white. The general trend is for a darker pigmentation the farther north the species is found. The slug covers itself in a thick foul-tasting mucus which serves as both protection against predators as well as a measure to keep moist. It is somewhat difficult to wash off.
This species is found in northern Europe (including Britain) and the Pacific Northwest