Abstract: Virulence and drug sensitivity of six Flavobacterium columnare isolates recovered from different fish species of Bangladesh were compared. Immersion method was employed to infect climbing perch (Anabus testudineus) using three different concentrations of bacteria viz. 1x 108, 1x 107 and 1x 106 colony forming units (cfu)/ml. The percentage infection and mortality caused by each isolate varied from 0-100% indicating wide variations in virulence. Isolates F03 and F32 appeared highly virulent in all challenge doses. At the highest challenge dose of 1x 108 cfu/ml, these two isolates were able to kill 100% fish by the end of the trial at day 7 post-challenge. While they killed only 10 and 30% fish respectively, at the lowest dose of 1x 106 cfu/ml during this period. Infection in fish due to challenge with these isolates started to appear by day 3 post-challenge. Isolates F10 and F35 were considered as medium virulent since they were able to kill 50 and 60%, and 20 and 40% fish at 1x 108 and 1x107 cfu/ml respectively, at the end of the trial. These two isolates were unable either to infect or kill any fish at 1x 106 cfu/ml. The other two isolates F16 and F25, appeared low or non-virulent. All the six isolates were found sensitive to six selected antibiotic substances appart from isolate F16, which was only sensitive to sulphaurethoxazole.