Streptococcus iniae and S. agalactiae are Gram-positive bacterial pathogens of cultured and wild fish. They are spherical or ovoid in shape and 0.5-2.0 μm in diameter. They occur in pairs or chains when grown in liquid media, are non-motile and non-spore-forming.

They are facultatively anaerobic, requiring nutritionally rich media for growth, and commonly attack red blood cell to produce greenish discolouration (α-haemolysis) or complete clearing (β-haemolysis) on blood agar. Both bacteria may present zoonotic concerns.