Abstract: The biological characteristics of bacterium Flavobacterium columnare were studied by Gram stain,colorimetry and agar diffusion methods.The bacterium cultured in broth Shieh meddium for 24 hours was showed different shape in the appearance including arch shape,U form,V form or S form and so on.Some of Flavobacterium columnare cultured for 72 hours appeared as branched cells and more granular involution form than that for 48 hours.The extracellular products(ECP) were found to be positive in the protease,lipoidase and lecithinase,but negative in amylase,gelatinase and urease.