This poorly known family only holds three described species: Ilibia ilbi in Australia, Ilibia mariana in the Marianas and Pseudoilbia lineata in New Zealand.
In external appearance members of this family are quite similar to those of the Runcinidae excepting they do not possess a gill or any shell remnant.
They are small and the body is brightly coloured with a smooth, slender and flattened slug-like appearance. Their notum is undivided and overhangs all around. The head shield and visceral mass merge without division. The eye spots are conspicuous dorsally.
The foot lacks any kind of tentacular or parapodial expansion but may have a ventral posterior furrow. The tail is longer than the notum.