An organism, measuring 0.86±0.32×0.63 ±0.24 μm, likely to be member of family Rickettsiaceae, was detected for the first time from an outbreak of a disease with mass mortality among pond-reared tilapia in Taiwan. The pathogen was demonstrated to be Gram-negative and stained positively with Liu's solution and to grow on To₂ and EPC cell cultures but not on nine artificial media selected and the pathogenicity was confirmed by Koch's postulates. Haematocrit in affected fish was decreased to 16.1±5.2% versus 28.3±5.1% for normal value. Typical lesions in moribund fish included marked white nodules and microscopical granulomatous formation all over the organs. The spleen enlarged to 5-50 times of the normal organ. Horizontal transmission without parasite vectors occurred. The experimental infection showed higher mortality at 15°C than at 30°C. In vitro experiments revealed that oxytetracycline which is legally permitted would probably be an effective antibiotic to control this disease. A field test showed that oral administration of oxytetracycline at a dose of 30-50 mg per kg body weight for 10-14 days was effective to control the disease. Clinical signs, susceptibility of various fishes to the pathogen and transmission electron-microscopy findings are described.