A total of 5000 samples were examined for Listeria monocytogenes, and 117 strains were isolated. Of these, 36 strains were selected according to their low virulence [for experimental animals]. An increasing virulence was observed when they were cultured on optimal media at 4.degree. C, with inoculations at every 3rd wk. This increased virulence, caused by temperature factors, can have an important role in the epidemiology of listeriosis [human].