Oxolinic acid (1-ethyl-1, 4-dihydro-6, 7-methylenedioxy-4-oxo-3-quinolinecarboxylic acid) has been known to possess an antibacterial activity against gram-negative bacteria (Aerobacter, Brucella, Pasteurella, etc.) and chemotherapeutic effects on infected animals. We also found that this compound had a superior antibacterial activity against several bacteria (Aeromonas, Vibrio, and Chondrococcus) isolated from diseased fishes, and that it was also active against Aeromonas liquefaciens (R+). Oxolinic acid exhibited a chemotherapeutic effect on carps infected with Aeromonas liquefaciens when it was orally administered (3mg/kg, once a day for 5 days) or when the fishes were bathed in medicated water (1mcg/ml, for 24 hours). The bathing method was also effective in loaches infected with Chondrococcus columnaris. Single oral doses of 40, 20, 10, and 5mg/kg in carps resulted in peak serum levels of 2.23 (15 hours after administration), 0.89 (24 hours after administration), 0.59 (24 hours after administration), and 0.73mcg/ml (24 hours after administration). Oral acute toxicity (LD50) was over 4, 000mg/kg in carp.