The susceptibilities to arsenic and cadmium together with the detection of plasmid DNA were evaluated for use as epidemiological markers for the subtyping of Listeria monocytogenes. Plasmid DNA was detected in 34% of 322 apparently unrelated isolates of L. monocytogenes. The resistance to cadmium and arsenic differentiated 565 apparently unrelated cultures into four groups, the smallest being 5% of cultures resistant to both agents, and the largest (53%) being sensitive to cadmium and resistant to arsenic. The resistance patterns to these agents and the presence of plasmid DNA varied markedly between the serotypes of the cultures. The detection of plasmid DNA was strongly associated with cadmium resistance in serogroup 1/2 cultures, but not within those of serogroup 4. Arsenic resistance was not associated with plasmid DNA. All methods were sufficiently stable to be useful for epidemiology investigations. The techniques described here offer simple methods which can be easily utilized in laboratories without a specialized expertise for this bacterium.