Streptococcosis is one of the most important bacterial  diseases in cold water fish because of its rapid outbreak specially in summer, relatively high morbidity and mortality, productive reduction and economic losses in aquaculture industry. However, it would be necessary to determine of 50% lethal  dose (LD50) of bacteria (Streptococcus faecium) and studying its acute effects on hematological and histopathological parameters. For measuring LD50 five dilutions of bacteria were obtained by Macfarlane method and the fish were injected by intraperitoneal (IP). The experience was carried out in 6 treatments and 3 replicates. The fish were injected by LD50 dose and after 10 days, samples of blood and tissues including: Eyes, brain, kidney, liver and gills were collected. The injected fish showed darkening of body, exophthalmia, abdominal distension and prolapsed anal. The  obtained  results  showed significant reduction (P<0.05) in level of red blood cells (RBC), hemoglobin (Hb), hematocrit (Hct) and significant increase (P<0.05) of mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) in test group compare to control group. In addition histopathological findings including hemorrhage in brain, eyes, kidney and liver, congestion in liver, brain and gills, inflammation of gills, meningitis and separation of menangial layers, presence of melanin pigments and increasing of mucus cells in the comial epithelium and were showed shrinkage, bowmans space dilation,  degeneration  and vacoelation in tubular cells, increasing of melanomacrofage centers and melanin pigments in kidney.