A typical fish louse of the genus Argulus 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.

[4] It attaches to its host, usually a fish, via its suction cups, pierces the skin with its sharp stylet, and feeds on blood.

[6] A heavy infestation causes inflammation of the skin, open hemorrhaging wounds, increased production of mucus, loss of scales, and corrosion of the fins.

Infested fish may exhibit loss of appetite and slowed growth, and behavioral signs such as erratic swimming and rubbing up against aquarium walls.

[7] The common fish louse is also a vector for pathogens, introducing organisms such as bacteria, flagellates, and the virus that causes spring viraemia of carp.

[10] The newly hatched larva can parasitize a host, attaching to it with its hooked antennae because it lacks suction cups.

[11] Food and sport fish and other commercially important species parasitized include carp and minnows such as goldfish and koi, members of the sunfish family, and salmonids such as salmon and trout.

[4] Hosts include blue bream (Ballerus ballerus), silver bream (Blicca bjoerkna), European eel (Anguilla anguilla), northern pike (Esox lucius), three-spined stickleback (Gasterosteus aculeatus), pumpkinseed (Lepomis gibbosus), ide (Leuciscus idus), abu mullet (Liza abu), European perch (Perca fluviatilis), common roach (Rutilus rutilus), common rudd (Scardinius erythropthalmus), wels catfish (Silurus glanis), zander (Sander lucioperca), tench (Tinca tinca), and Atlantic horse mackerel (Trachurus trachurus).

Major outbreaks in rainbow trout fisheries in the United Kingdom have resulted in total losses.

[7] Carp aquaculture in Russia has experienced infestations in which fish were coated in "several hundred" parasites before dying.