Listeria monocytogenes is divided into serovars 1-7 (including subserovars) which belong to several genospecies as determined by gene homology studies. At least two of the five genomic species of L. monocytogenes are avirulent and cannot be separated by serological analysis from virulent strains. During the past two decades, several thousand Listeria strains have been analysed by the reference laboratory of the Institute of Hygiene in Würzburg, Germany. A study of the origin of the strains and their association with disease in animals and man in conjunction with biochemical and serological methods, allowed the separation of at least one group which is apathogenic and non-virulent to laboratory animals under ordinary test conditions. This apathogenic species, L. innocua should be separated from L. monocytogenes when Listeria-like strains are isolated from environmental sources.