The mucosal surfaces of fish, including skin, gill and gastrointestinal tract, contain numerous poorly studied immune substances that act as the first line of defence against invading pathogens.The fish body’s mucosal surfaces are protected from pathogens, physical and chemical attacks by the gel-like extracellular matrix (mucus) which is synthesized, stored and secreted by mucous cells. In order to better understand the vaccine-induced immune responses of mucous cells, histological and histochemical techniques were used to investigate the dynamic changes of the numbers and types of mucous cells in mucosa-associated lymphoid tissues, including skin, gills, foregut, midgut and hindgut, in Paralichthys olivaceus after immersion immunization with inactivated Edwardsiella tarda. The results of hematoxylin and eosin (HE) staining, alcian blue and periodic acid Schiff reaction (AB-PAS), indicated that the type Ⅰ mucous cells containing PAS-positive neutral mucopolysaccharide and type Ⅲ cells containing neutral mucopolysaccharide and a little AB-positive acidic mucopolysaccharide in skin, type Ⅰ, Ⅲ, and Ⅳwhich contained acidic mucopolysaccharide and a little neutral mucopolysaccharide in gill, type Ⅰ in foregut mainly, and type Ⅰ, Ⅲ, Ⅳ and Ⅱ which contained acidic mucopolysaccharide in midgut and hindgut, were observed before immunization. After immunization with inactivated E. tarda, the mucous cell numbers were increased over time and were peaked on day 3 post immunization (p.i.) in skin, gill, foregut and midgut and on day 5 p.i. in hindgut; and then were decreased to the level of control groups; Additionally, the amount of typeⅠmucous cells was reduced while the amount of type Ⅱ, Ⅲ and Ⅳ mucous cells was increased, revealing that the components of mucous cells have been changed from neutral mucopolysaccharide to acidic mucopolysaccharide due to the immunization with E. tarda. The results of AB staining showed that the mucous cells containing acidic mucopolysaccharide were increased after immunization, which were mostly sulfated acid mucopolysaccharide-containing mucous cells. These results will provide valuable information for the researches on the mucosal immunity in fish.