Atlantic salmon, Salmo salar, is normally rather refractive to infection with the ectoparasitic monogenean Gyrodactylus derjavini but dexamethasone treatment of the host increases the susceptibility. The causative mechanisms were elucidated in this work. Groups of Atlantic salmon were treated by intra-peritoneal dexamethasone injections and subsequently infected with G. derjavini. It was shown that both the infection level and the mucous cell density of caudal and pelvic fins were affected by the treatment. Significantly higher mucous cell densities were found on infected and treated fish whereas non-infected and treated fish showed no significant elevation of cell density. This suggests that mucous cell discharge elicited by infection is inhibited by the drug. The association with elevated parasite counts in these fish can be explained either by decreased anti-parasitic mucus action or by parasite predilection for intact mucous cells.