Flavobacterium columnare (Flexibacter columnaris) is an important cause of gill and skin disease in freshwater fish species, often causing high mortality. In previous studies, virulence of F. columnare was correlated with the ability to adhere to the gill tissue. To gain insight into the factors responsible for adherence, a gill perfusion model was used. The bacterial cells of the high virulence strain AJS 1 were exposed to various treatments, after which they were added to the organ bath of an isolated gill arch and adherence to the gill tissue assessed. Adherence capabilities were significantly reduced following treatment of the bacteria with sodium metaperiodate or incubating them with d-glucose, N-acetyl-d-glucosamine, d-galactose and d-sucrose. Incubation of the bacteria with trypsin and pronase did not significantly inhibit adherence. The binding sites for F. columnare on the gill tissue were also partially characterised. Treatment of the gill with sodium metaperiodate reduced adhesion, but treatment with pronase or trypsin did not cause any significant reduction, indicating that the major component of the receptor is of carbohydrate nature. Adherence ability of the bacteria correlated well with their haemagglutination capacity using chicken and guinea pig erythrocytes. Higher haemagglutination titres were obtained with the highly virulent strain AJS 1 than with strain AJS 4, a strain with low virulence and adherence capacity. Haemagglutination was partially inhibited after incubation of the bacteria with d-glucose and N-acetyl-d-glucosamine and after treatment of the bacteria at 41_°C for 10_min (minor heat treatment). It was completely abolished following incubation of the bacterial cells with sodium metaperiodate and intensive heat treatment (65_°C, 25_min). Haemagglutination was also in-sensitive to pronase and trypsin treatment. Transmission electron microscopy (TEM) revealed that the high virulence strain had a thick capsule (120–130_nm) with a regular, dense appearance, whereas the capsule of the low virulence strain was much thinner (80–90_nm) and less dense. TEM also demonstrated the loss of the capsule of the high virulence strain after treatment of the bacterial cells with minor heat and sodium metaperiodate. These results indicate that a lectin-like carbohydrate-binding substance incorporated in the capsule is responsible for the attachment of F. columnare to the gill tissue.