Two previously isolated phages were used to examine the therapeutic effects against Pseudomonas plecoglossicida infection in ayu Plecoglossus altivelis. Phage PPp-W4 (Podoviridae) inhibited the in vitro growth of P. plecoglossicida more effectively than Phage PPpW-3 (Myoviridae), and a mixture (PPpW-3/W-4) of the 2 phages exhibited the highest inhibitory activity. In phage therapy experiments, ayu were fed P. plecoglossicida-impregnated feed (10(7) CFU fish(-1)) and then fed phage-impregnated feed (10(7) PFU fish(-1)). Mortalities of fish receiving PPpW-3, PPpW-4, PPpW-3/W-4, and a control fish receiving no phages were 53.3, 40.0, 20.0 and 93.3%, respectively. Phage (PPpW-3/W-4)-receiving fish also showed high protection against water-borne infection with P. plecoglossicida. In a field trial, when phage (PPpW-3/W-4)-impregnated feed was administered to ayu in a pond where the disease occurred naturally, daily mortality of fish decreased at a constant level (5% d(-1)) to one-third after a 2 wk period. The causal relationship of phages in this phenomenon was verified by the long-lasting appearance of administered phages in the kidneys of the fish, and a disappearance of P. plecoglossicida from apparently healthy fish. Neither phage-resistant organisms nor phage-neutralizing antibodies were detected in diseased fish or apparently healthy fish, respectively. These results indicate the potential for phage control of the disease.