Summary 1. Dysentery in Southern California is caused primarily by three types of microörganisms, Shigella paradysenteriae, Salmonella morgani, together with several other species of Salmonella and Endamoeba histolytica. 2. Acute bacillary dysentery is encountered much more frequently than acute amebic dysentery. These two types may often be differentiated by microscopic examination of the stool, the bacterial type usually showing a prolific cellular exudate with a preponderance of polymorphonuclear leucocytes and the amebic type showing a scanty exudate made up almost entirely of macrophage cells in addition to the trophozoites of amebae. 3. Chronic amebiasis is proportionately more common than chronic bacillary dysentery. The latter, however, occurs more frequently than many suspect and should be suspected as being associated with Shigella or Salmonella rather than with Bargen's Streptococcus. 4. Carriers of E. histolytica, Shigella and Salmonella have been observed, the bacterial types being somewhat less common than the amebic. 5. Recently an outbreak of bacillary dysentery occurred which was caused by the Shigella paradysenteriae “Sonné.” 6. Although the agglutination reactions are diagnostic, the lactose fermentation is much slower and less regular in occurrence than such reactions commonly reported for “Sonné” and also less regular than those produced by five other strains of “Sonné” with which they were compared. 7. Fermentation occurs first under anaerobic conditions, either in the Durham tubes of lactose broth medium or at the bottom of a lactose agar slant. Fermentation, in media in which the Durham tube is not present, occurs more frequently in nutrient broth medium, than in dibasic phosphate peptone medium. 8. Rough and smooth colonies and “satellite” colonies may give rise either to rapid or to slow lactose fermentation. 9. These variations from the usual descriptions given for the “Sonné” type of organism should be considered in the laboratory diagnosis of dysenteric bacteria for certain non-lactose fermenting strains which fail to agglutinate with the common antisera, may be found to belong to the “Sonné” type.