Serious Plesiomonas (Aeromonas) shigelloides infections have rarely been reported, and have probably been missed because this organism is very similar to the Enterobacteriaceae in associated clinical disease, and in properties investigated in the diagnostic laboratory. A case of overwhelming neonatal meningitis and sepsis is discussed, and the use of the simple indophenol oxidase test on laboratory isolates of gram-negative rods is urged to distinguish this organism and its close relatives from the Enterobacteriaceae.