This is a parasite of many fish species and grows up to 7 millimeters long by 5 millimeters wide. It is very flat with an oval or rounded carapace, two compound eyes, sucking mouthparts with a piercing stylet, and two suction cups it uses to attach to its host. These "suctorial organs" are the first of its two pairs of maxillae, modified in shape. Its paired appendages have hooks and spines, and are used for swimming. The female is larger than the male and has a visible pair of spermathecae on its posterior end, in which the male deposits sperm.

Argulus foliaceus is found in warm, eutrophic, still lakes of both fresh and brackish water.

All year

It attaches to its host, usually a fish, via its suction cups, pierces the skin with its sharp stylet, and feeds on blood. It may also live in the gills.

Common and widespread but predominently in the south.

Probably common but under-recorded.