BACKGROUND: Streptococcosis caused by Streptococcus iniae is one the most important bacterial diseases in aquaculture industry worldwide. Objectives: This study was aimed to assess the experimental pathogenicity of Streptococcus iniae in Persian sturgeon (Acipenser persicus) fingerling. Methods: A number of 400 Persian sturgeon (Acipenser persicus) fingerling weighting 17±3 g were used. Fish were challenged with a virulent strain of Streptococcus iniae via both intraperitoneal and intramuscular injections at dosage of 4.7×106, 4.7×105 , 4.7×104, 4.7×103, 4.7×102 cells/fish. Each treatment group included 12 fish in two replicates. Control fish received 0.1 mL per fish sterile normal saline (0.9% NaCl). Results: Clinically mortality started after 24 hours post-challenge and the affected fish showed listless, spiral swimming, spot haemorrhages on different parts of bodies particularly at the base of fins, the lateral line around the bone columns, on the base of barbells, mouth and around anal area. Also signs of abdominal distention, hyperemia of intestine, accumulation of bloody fluid in abdominal cavity, lordosis and scoliosis as well as hemorrhages in eyes were seen. The lethal concentration (LD50) of intraperitoneal injection was calculated 1.1×103, 8×103, 3.7×106 cells/fish after 48, 72 and 96 hours post-challenge, respectively. The LD50 of intramuscular injection was 4.8×102, 1.8×103 and 6.4×105 cells/fish at 48, 72 and 96 hours post-challenge, respectively (p<0.05). No mortality or abnormal signs was seen in control fish up to 14 days post-experiment. Conclusions: The results of this study showed that Parisian sturgeon fingerling is highly susceptible to Streptococcosis.