Juvenile Japanese flounder Paralichthys olivaceus were immersion-challenged (2.0 x 10(6) CFU/mL, 60 min) with Edwardsiella tarda NUF806 strain. From the following day, diets containing fosfomycin (FOM) calcium were administrated orally at concentrations of 20, 40 or 80 mg potency/kg body weight/day for 6 consecutive days. Cumulative mortality of fish during the observation period of 21 days after the drug administration was significantly lower in all groups of FOM-treated fish than that in the control group. In addition, NUF806 strain and recent clinical isolates (n = 30) of E. tarda examined were all susceptible to FOM, with MIC of 2-4 mu g/mL when estimated using the glucose-6-phosphate-containing Mueller-Hinton broth. The results suggest that FOM is a promising candidate for the treatment of edwardsiellosis in cultured Japanese flounder.