The multiplication and distribution of an atypical Aeromonas salmonicida strain, which was isolated as a causative agent of "head ulcer disease" of cultured eels, in artificially infected Japanese eels (Anguilla japonica) were studied by bacteriological culture technique. When the eels were injected intramuscularly with a lethal dose of the atypical A. salmonicida strain at 20.degree. C, the pathogen was recovered at high concentrations (107-109 CFU/g) from the muscle of the injected site throughout the course of infection. But in the spleen and kidney very small numbers of the pathogen were detected even at the moribund stage, and the blood and brain were almost aseptic. A similar result was obtained in the eels which were artifically injured on the skin and infected with the pathogen. These data indicate that the disesae does not attain to distinct septicemic condition but the local proliferation of the pathogen will be the death of the host fish, and this pathological feature is in a striking contrast to other bacterial infections of eels. It was also found that the bacterial growth was significantly inhibited both in vitro and in vivo at 30.degree.T C, which suggests that temperature manipulation of pond water will be an effective measure to cotrol the disease.