Aeromonas hydrophila and Flavobacterium columnare, the aetiological agents of motile aeromonas septicaemia (MAS) and columnaris disease respectively, have been recently causing crippling mortalities to the sunshine bass, Morone chrysops female ×Morone saxatilis male (Percichthyidae), industry in the United States. Isolates of A. hydrophila and F. columnare obtained from fish that died during farm outbreaks were subjected to in vitro evaluation of florfenicol (FFC), copper sulphate (CuSO4) and potassium permanganate (KMnO4). Florfenicol inhibited the growth of A. hydrophila and F. columnare more than CuSO4 and KMnO4. The minimum inhibition concentration (MIC) of FFC was 0.04 ± 0 and 0.2 ± 0.1 mg L−1 for A. hydrophila and F. columnare respectively, while the 50% inhibition concentration (IC50) for A. hydrophila and F. columnare was 0.23 ± 0.01 and 0.4 ± 0.2 mg L−1 respectively. Copper sulphate was more effective against A. hydrophila than KMnO4; CuSO4 had a MIC of 83.2 ± 0 mg L−1 compared to 158.0 ± 0 mg L−1 for KMnO4. Copper sulphate was also more effective against F. columnare than KMnO4. The IC50 values of CuSO4 and KMnO4 towards F. columnare were 4.8 ± 0.3 and 8.7 ± 1.6 mg L−1 respectively, and the minimum bactericidal concentration values of CuSO4 and KMnO4 towards F. columnare were 25.0 ± 0 and > 158.0 mg L−1 respectively. In addition, F. columnare was more sensitive to CuSO4 and KMnO4 than A. hydrophila.