A bacterial meningoencephalitis in St. Peter's fish (Tilapia spp.) and trout (Oncorhynchus mykiss) appeared in Israel in 1986 and rapidly spread throughout the country, causing considerable economic losses. We isolated and identified the agents of this disease. They were Gram-positive, nonsporulating, facultatively anaerobic chain-forming cocci, catalase negative. They were able to grow at pH 9.6 but not at 10 degrees C nor at 45 degrees C nor in the presence of 40% (vol/vol) bile salts or in the presence of 6.5% NaCl (wt/vol). DNA base composition (G + C% = 37%), DNA-DNA hybridizations, biochemical and serological studies indicated that these strains constitute two new distinct species of streptococci that we propose to name Streptooccus shiloi (type strain ND 2-16) and Streptococcus difficile (type strain ND 2-22). Unclassified isolates from Japan and from Taiwan belonged to Streptococcus shiloi or Streptococcus difficile, showing the cosmopolitan distribution of these two newly described species.