Tandonia budapestensis is a species of air-breathing, keeled, land slug, a shell-less terrestrial gastropod mollusks in the family Milacidae.

[4] This slug is basically dark brownish grey, with a keel of a brighter color.

The body is yellowish-grey to brown or dark grey, with numerous black spots, so that the slug may appear to be evenly black-brown, slightly lighter at the sides.

The body mucus is usually colourless, thick and sticky but it is yellowish when the slug is irritated.

[3] It occurs in the following countries, amongst others: Europe: other countries: This species is already established in the USA, and is considered to represent a potentially serious threat as a pest, an invasive species which could negatively affect agriculture, natural ecosystems, human health or commerce.

[11] The habitat of Tandonia budapestensis includes parks, gardens, ruins and cultivated fields.

[9] In Britain the copulation of this slug species takes place from November to January; in Central Europe from April to autumn.

[3] Copulation begins usually at night and may last 15 hours or more; everted genitalia are visible between the partners.