Studies were performed on a "gliding bacteria" associated with the disease of sheatfish (S. glanis) fry reared in a recirculation system. The susceptibility of these bacteria to antibiotics was also examined. It is suggested that the strain isolated was Flexibacter columnaris. Out of the 25 antibiotics tested, there were many to which the bacterium proved to be sensitive. Polymixin- beta resistance characteristic of Flexibacter columnaris was evident in this case. The observations showed that with a heavily infected population treatment with antibiotics would not be effective and preventive measures seem to be more promising in the fight against the pathogen.