Fish: During spring and summer 2011, sampling from 14 rainbow trout (O. mykiss) farms in Kohgiluyeh and Buyer Ahmad Province (located in North West of Iran) was done. 280 rainbow trout (20 fish from each farm, 10-200 g each) suspected of having streptococcosis/lactococcosis from these farms with a water temperature of 15-19°C were examined for the presence of the etiological agent of the disease. Moribund fish with external and internal body hemorrhages, abdominal distension and exophthalmia were euthanized with a blow to the head and used immediately for bacteriological examination.