Characteristics, pathogenicity and control of the causative organisms isolated front diseased cultured flounder, Paralichthys olivaceus were studied. The causative organisms were identified as E tarda by biochemical and biophysical characteristics. Also, it strains were named as E tarda KBF-1 and E tarda KMF-1, and optimal pH of E tarda KBF-1 and E tarda KMF-1 were 8.0, and optimal concentration of NaCl. E tarda KBF-1 was 0% and E tarda KMF-1 was 1%. In the pathogenicity test, 0~10 of the flounders of artificially infected group(E tarda KBF-1) with $1.0{ imes}10^7$ cfu/fish were died within 60 hrs, but 0~9 flounders infected group with 41.0{ imes}10^6$ cfu/fish were died within 60 hrs. Also, 0~10 flounders infected group(E tarda KMF-1) with $1.0{ imes}10^7$ cfu/fish were died within 36 hrs, while 0~7 flounders infected with $1.0{ imes}10^6$ cfu/fish were died within 60 hrs. Drug sensitivity of E tarda KBF-1 strain was resistant to AM, CF and N, and intermediate to E, K and S, and sensitivity to C, G, SxT and FF. But E tarda KMF-1 strain was resistant to CF, E and V, and intermediate to AM, C, N and SxT, and sensitivity to GM and FF.