Multiple drug resistant gram-negative bacteria were isolated from the intestinal tracts of cultured carps and also from the water of five carp-ponds at Shioda Town in Nagano Prefecture, during the period from May to October, 1970. The isolated bacteria were Aeromonas liquefaciens, Pseudomonas, Enterobacteriaceae and Vibrio. Transferable R factors were detected in 189 of 425 strains of A. liquefaciens and 2 of 25 strains of Enterobacteriaceae. These R factors were classified into fi- type and most of them (91.6%) were resistant to both sulfonamides (SA) and tetracycline (TC).During the sampling period, there were epidemics of bacterial infection of carp in 3 of the 5 ponds investigated and sulfonamides were used for treatment. Probably due to this treatment, a marked increase in SA-resistant (grown on nutrient agar containing 500μg/ml of SA) and TC-resistant (grown on nutrient agar containing 25μg/ml of TC) bacteria carrying R factors in the intestinal tracts of carp was recognized.